Jamie, I dont know your gear but if you are using a mixer you can certainly use a plug-in for playing live. You would just need to send the signal you were recording back to your monitors (headphones or speakers).
Personally I am trying to use plug-ins for recording (to keep everything digital) while I use an old Peavy rack effects just for putting some reverb on the monitor siginal, because my singer also likes to hear his voice "wet". It gives him the illusion that he is singing better than he is!