I don't think the OP was confused, he just wanted them to do more with the character that had served with him and develop him more into the plot. He could have been a loyal ally, having served under Maximus previously, but I don't think they do anything with him after the reenactment of Carthage.

Vindobona was a Celtic settlement and later a Roman military camp on the site of the modern city of Vienna in Austria. Around 15 BC, the kingdom of Noricum was included in the Roman Empire. Henceforth, the Danube marked the border of the empire, and the Romans built fortifications and settlements on the banks of the Danube, including Vindobona with an estimated population of 15,000
nothing to explain, he served in battle under the emperor and the general.

Apologies to the OP, my mistake.
This means something, this is important.

Correct Sithlord, it's mentioned on two occasions.
The Heroes Journey -

Yes, Cicero spoke that line to Lucilla, but I think the OP was referring to the line spoken by a gladiator to Maximus when they were just about to reenact the massacre of Carthage.

"I served with you at Vindabona"
I thought that line was spoken by Cicero to Lucilla when he stopped her carriage and tried to arrange the meeting between her and Maximus. I think the line was actually "I served the general at Vindabona, I serve him still"
That's how I understood it, no confusion there.
This means something, this is important.

He said 4000 and 2000, "that's 5000 for an old friend", in hopes of bartering for a better deal after the previous sale wasn't what it should've been.

It kind of goes nowhere but I am more confused by Proximo's math when he's buying Maximus and the other slaves and the beasts. 4000 + 2000 = 5000? Unless he's intentionally jacking the salesman around, which I guess is possible since he was pissed about the queer giraffes.

"I served with you at Vindabona"
I felt there should have been more exposition on this.
Maybe they were waiters together in high school (?) at a
Vindabona Caf,
or something?
.