Let's give someone else a chance at these roles, shall we? Vin Diesel was excellent here, and two hours ago I'd have been surprised to see those words coming from my keyboard.

No. Vin Diesel was perfect, he reminded me of a young Marlon Brando (who he has said is his favorite)
the truth is we can't all be stars, but I'll be god damned if I'll settle for bronze.

Pesci has done gangster and he's done courtrooms so why combine them, use your imagination.

OP said, "With Pesci, would've been better."
even though i think Pesci plays those gangster roles great (i.e. Casino/Goodfellas) i think Diesel fit this movie better than Pesci would have, especially since Pesci is getting up there in age nowadays.

Pesci doesn't have the physicality to play Jackie D (who okayed Diesel to play him).
Diesel did a surprisingly good job with this role.

Have to agree with larpine. I think you maybe thought some of the lines sounded bad because you knew it was Vin Diesel.
I didn't even realise it was him and I thought the character was perfect. I think Pesci has been too typecast into these roles and I'm not sure that you wouldn't just think back to his character in other roles.
Also this character required someone who looked a bit fragile. A bit emotional and a bit naive and I think Vin did a stirling job. I really felt for him when he was getting snubbed by all his collegues/friends and he stayed loyal. He was almost a bit dilusional believing everyone really loved him when in the end they didn't care as much as he thought. I just don't think Pesci could have done the character in the same way.
So thats my 2 cents

With respect, need to disagree. Diesel was a really nice surprise where as with Pesci, it would have been typecasting as this stage of his career.