For those who do live gigs- what would be the routing, if I wanted to use a vocal plug in on a laptop. I think this is doable : Such that somewhere between my vocal mic and the PA, I can use a laptop based fx - or no?
 
thanks !  

Sure it can be done.... a lot of touring engineers use plugin effects these days.
 
The number of discrete effects you can run will be determined by the number of ins/outs on your interface - and the number of sends/returns on your mixing board.
 
In you case - you are asking about a vocal plugin on the vocal only.
1: If the board has a channel insert - use that to connect the interface.
2: Or you can use a send/return to connect to the interface.
 
Although it may be possible, I do not recommend plugging the mic into the laptop, then to the console. Try to treat the laptop (and inteface) the way you would outboard effects. Just my 2 cents...