Can you play RR to roll in another dice for your vehicle? Wasn’t exactly sure with this card and wanted to verify how it works.

In general you activate the vehicle and later resolve the dice or perhaps have it removed.Thus, you'll have an exhausted vehicle with a dice on it. At that point you can now roll the dice into your pool again, however, if you get a blank you'll discard the vehicle and dice that you just rolled.

You could not grab a second copy of that dice and roll it in.

Well the wording says it all I think:

"Choose a die ON..."

The die must be on the card, not in the pool.

Chak