VP is a pick done for 4 reasons only:
solid for president if the president has something happen to him (severe mental problems, dies, etc)
break votes in the senate in a positive manner for Dem causes in case of tie
gives the president access to an additional voting bloc they wouldn't have had otherwise.
they alongside the CoS bring national security and foreign relations to the forefront with the President
Different Presidential campaigns will weigh these benefits differently depending on their candidate, but generally you're looking for someone who can excel at 2-3 of these things.
Take Obama/Biden for example:
While Biden wouldn't have been an optimal president to replace Obama, the main reason he was chosen was because he secured Obama an extremely large voting bloc he wouldn't have had otherwise (old white men, more conservative independents) and is adept at navigating national security & foreign relations if nothing else.
But that's why I don't think Pete is a good VP pick for Bernie, because I legit can't think of 2 of those things he would excel at. Solid presidential replacement for Bernie? IMO no. Does he offer Bernie a voting bloc he doesn't already have? Nope. Is he capable of excelling at national security/foreign relations when he's just so green? Also no.
I think Sanders would want to look at someone like Warren for VP. Warren would be good at basically all 4 of those things in relation to Bernie:
good presidential replacement? Sure she's basically bernie-lite.
can break senate votes properly
while Bernie has a lot of younger adult women grabbed, adding Warren would help entice the older woman voting bloc
she's solid for foreign relations and national security
Honestly the only downside to her is that Massachusetts already heavily leans Democrat so adding a VP from there doesn't help entice an entire state as a voting bloc, whereas a hypothetical O'Rourke could help secure Texas for example.
But I definitely feel like Sander's campaign would pretty much discard Pete as an option just for not bringing much to the table for their administration really.