To summarise:
1. You can perform a Flat Out move in the Scouting "phase".
2. If you have first turn, the vehicle may act as normal. The flat out movement from Scouting will not give you a cover save in your opponent's first turn.
3. If you have second turn, the vehicle will get a 4+ cover save in your opponent's first turn.
This is all clarified neatly in the Rulebook
FAQ
I always move my Valkyries/Vendettas flat out during their scout move if they're starting on the table, even if they end up still in my deployment zone, just for that 4+ cover save if my opponent gets first turn