Calling 911? Ving Rhames would've torn him 2 new beep for starters.
"It was the best of times; it was the blurst of times."

Well yes, the
Trainspotting
solution would not work here. Different society, different laws.
Vincent's character walks a tightrope in this film between charisma and dumbness. Still, the scene where he deals with the OD was one of the best film scenes of the 1990s.
"Chicken soup - with a beep straw."

Unless you are prepared to have Mia go to jail and royally piss off her husband, your boss, none of these are good solutions. remember, the movie takes place in mid 90s California and laws were different back then. That's what Lance means by bite the bullet and get a lawyer. Police were notified on OD cases back then by the laws of most states.
Probably a really bad idea to screw up the one easy job you have over the weekend when your Boss' wife is on the line.

In the Scottish film
Trainspotting
, one character ODs or has a bad trip while visiting a drug dealer. The drug dealer phones a taxi, puts the ODing character out on the street, loads him into the cab when it arrives and the taxi driver drives the character to Accident and Emergency of the nearest hospital, after first taking some money from the ODing character's breast pocket for the fare (left there by the drug dealer). It is treated as almost a routine way to deal with an OD. But perhaps Vincent felt he had no such option with a leading hood's wife.
"Chicken soup - with a beep straw."

I think he felt that Lance could help him without the incident getting back to Marcellus. He would've ordered Vincent's death if Mia had died. If he took her to the hospital, Marcellus would've found out about it.
If Lance refused to help Vincent, Vincent probably would've told Marcellus about Lance not helping him and Marcellus would've killed him too.

Right on,
bohunkchicklet
.
The flat-brimmed baseball cap is the modern-day dunce cap.

Mia would tell him and he was on camera at their house. This was also after 1am. If Vega hadn't shown up, her hubby would have known. Mia would have called him and said that he never showed up.
And Wallace isn't stupid. Too many coincidences. Vincent just so happens to defy an order from his boss by not showing up or arriving at the house and not calling to say that his wife was missing?
There's no way that Vincent could have come up with a believable story. Not to mention the fact that we would have missed all those comedic gems.
"Prank caller, prank caller"
"I gotta stab her 3 times?"
"The day that I bring an od-ing bitch over to your house, then I give her the shot"
"That was f##king trippy"
"If it's so important than why don't you keep it with the shot?"
I could go on and on. Just another hilarious scene in this movie.
We would have missed out on all that if he had taken her to the hospital.

Thank you for all of your answers, maybe Vincent could have driven her to the ER and left her right outside of the main entrance to the ER section of the hospital, then one of the nurses, doctors or orderlies could walk outside to smoke a cigarette or an ambulance could arrive with someone else and just notice her lying there, take her inside and save her life.
Vincent could just tell Marsellous that he never saw her at the house and have the hospital contact her husband later on, that would save her life and get Vincent out of the mess, but Mr. Wallace might get into trouble.

Vincent seems to be an old friend but even so, Marsellus would have subjected him to some close questioning, and the true story might well have come out, resulting in anything between "grease spot" and "Zed's dead" status for Vincent. If M is feeling merciful, Vincent might die quickly.
When Butch fails to take the dive, M orders his hoods to take in Butch's coach for questioning even though he probably knew nothing about Butch's plans. His coach was in for a rough time, and I don't think his wife ODing is just going to be shrugged off by Marsellus. V probably handled the situation about as well as he could.
"Chicken soup - with a beep straw."

Take her to a payphone and anonymously call 911 then take off. Tell Marcellus that she didn't answer the door when he went to pick her up or better yet they went out and he dropped her off at home and that was the last that he saw of her.
Vincent would have to undergo some pretty intense questioning. I would not want to face a POed Marsellus in that situation. He seemed like a guy who was pretty adept at smelling BS.
Vincent probably made the best decision out of a list of very bad choices. In doing what he did, even if she dies, he at least made an honest effort to save her life. He'd have at least a slim chance of surviving.

This message has been deleted.

Take her to a payphone and anonymously call 911 then take off. Tell Marcellus that she didn't answer the door when he went to pick her up or better yet they went out and he dropped her off at home and that was the last that he saw of her.

My iMDB profile
http://www.imdb.com/board/24297325/?ref_=fn_al_nm_1

If Vince got into trouble with Marcellus, he was an effing grease spot.
"'Extremely High Voltage.' Well, I don't need safety gloves, because I'm Homer SimpsoZZZZZZZZZ" - Frank Grimes

Back in the mid 90s, most states had laws saying if someone was brought to the hospital after Oding the hospital was supposed to call the police. Ohio I know had that law and Im fairly sure California did too. This was the tail end of the tough on drugs era.
Then lawmakers realized that people were dying of overdosing and could have been saved. But they werent going to the hospital because they were afraid of getting into legal trouble.
So Vince was afraid of both the law and Marcellus.

Uhhh, wife of a gangster, doing illegal drugs, and oh, btw, he himself is a hitman, who probably has a warrant or two. Oh, yeah, he probably had some weed on him as well.
When we're thinking about our own brain, would that be a mental paradox??

Lance does tell Vince to bite the bullet and hire a lawyer, implying the hospital would be the ones to call the police.
And it wouldnt be impossible to drive Mia to the hospitals Emergency Room and say that Vincent came to her house and found her OD'd
Oh crap. So now the police would have probable cause that there are drugs in Marcelles Wallace's house? Yikes, Vince would definitely be a dead man.
You are sin.

He wasn't afraid of getting in trouble with the authorities; he was afraid of Marsellus. After all, it wasn't so much that she'd (almost) ODed but that she'd done so off of Vince's heroin which makes him DIRECTLY responsible for her near death. Lance would probably be in a similar boat since he was Vince's heroin supplier(i.e. Vince wouldn't have had the heroin for Mia to OD on if Lance hadn't sold it to him in the first place).
Living is easy with eyes closed
Misunderstanding all you see

Also what would Marsellous do if Lance had refused to help Vincent after Vincent showed up at his house with Mia?
Probably something to the effect of a blowtorch and a pair of pliers.
Now you agree with me!