Can we use the Vanguard for exploration?

Yeah, whatever you want.

In theory, a Sentinel would probably be the best of the Vanguards just because it'll have the biggest electronics kit with its giant avionics racks inside.

Fuel-wise and everything else?

It is a long-range fighter, so it should be able to go out and come back. Maybe not as great as a dedicated explorer, but having enough options in your ship is a huge thing.