Don't use this site. It hasn't been updated in well over a year and I don't think they are an official dealer. Also, that website was around before Frank to over and it has never been the official website for Vanguard. I was with the vanguard factory team last year and we had people ask us about this site and we were told by the west coast distributer to have vanguard users avoid it.
Now, if something has changed, then I give my apologies. However, I would ask one of the mastertechs or manufacturer reps for Vanguard about what sites are official vanguard dealers. They would know and would be able to direct you to were to get a new back cap from an authorized dealer so you have the company backing them and you.
Your best bet is to pm Reaper (ACI) and work with him. He is a Vanguard rep and is the best person to assist you.