Am I correct that my domestic flight departs/arrives at T3, while VS does it at T2 ?
Delta operates out of T2 and T3. VS is in T2.
Is there an easy passage within security from T3 to T2 on the way out ?
There's no airside walkway between T2 and T3 however Delta offers airside shuttles between T2, T3, and TBIT. Delta has a
useful map which explains the layout.
Unsure of T3 SC status, it was fairly crowded when I last transited back in November though I do recall a nice setup of grab 'n go style options (wraps, fruit, cheese cubes, etc.)