Can you attack with a vehicle that was crewed during your main phase but before combat?

Crewing a Vehicle (by tapping creatures with total power meeting the crew cost) causes it to become an artifact creature until end of turn. This is a continuous effect granted by the crew ability (CR 702.122). Once it is a creature, it is eligible to attack like any other creature.

The key timing rule is CR 509.1: during the Declare Attackers step, you declare which creatures you control will attack. Any creature you control that has been a creature since the beginning of your turn satisfies the 'has been controlled since the beginning of your turn' requirement — but Vehicles that are crewed mid-turn still qualify as long as they are creatures at the moment attackers are declared.

There is no rule requiring a permanent to have been a creature at the start of your turn in order to attack. The only relevant restriction is the summoning sickness rule (CR 302.6), which prevents a creature from attacking if it has not been under your control since your turn began — but this applies to the permanent, not just its creature status. If you've controlled the Vehicle since your turn began (or earlier), it is free from summoning sickness even when newly crewed.

Concrete example: You control a Smuggler's Copter that has been on the battlefield since last turn. During your pre-combat main phase you tap two 1/1 tokens to crew it (crew 1 requires 1 total power, easily met). When you move to combat and the Declare Attackers step begins, Smuggler's Copter is a 3/3 creature artifact and can be declared as an attacker.

If instead you played the Vehicle this very turn and it has summoning sickness, crewing it makes it a creature but it still cannot attack due to CR 302.6, because you have not controlled that permanent since the beginning of your turn.