I have never seen merch going on sale after concerts, so if you want something buy it as you get in most likely your size will be gone by the end of the show. (or the shirt you like).
I have never gotten VIP, but for this last show I got general admission, prepare to be there at least as soon as doors open, get to your spot and DO NOT MOVE. no peeing, no be right back (unless you have people with you saving your spot). Everybody wants the spot you are on, so stand your ground and don't give in. you will need the space to move your legs a little bit while you wait for Pendulum (amazing band by the way) and the previous band which is also cool.
Lucky for us most LP fans are really cool and are there to have a good time and not cause problems, so I had a great experience last night, I was there at the rail with my 12 and 16 year old nieces, and it was very cool.
Hope you enjoy the show too!