Depending on what area of the city you will be in, here are some:
-----West End----
Forest Lounge
124 Boundary St
*yummy pies and salads
----Brisbane City---
Govinda's
99 Elizabeth Street
*11.00 am - 3.00 pm Monday - Thursday
*5.00 pm - 7.30 pm Monday - Thursday
*11.00 am - 8.30 pm Friday
*11.00 am - 2.30 pm Saturday
*I've not been there but I've hard good things and it is supposed to be cheap
----Fortitude Valley----
Huo Fo Vegetarian Court
198 Wickham Street
*A little hole in the wall, be careful or you'll miss it!
*Very yummy and very cheap

.
*Not completely vegan I think so check before purchasing!
Also check out this link which lists all of the above and A LOT more:
http://www.vegsoc.org.au/restaurants_category.asp?RestaurantsCategory=Brisbane&vegetarian=False
Enjoy your concert! Who are you seeing?