be prepared to not get that Vault cap back on once it is removed.
I had to remove mine rather forcibly and it could not be re-used. Luckily, I contacted the folks at Vault and they set me up with a kit, which included a new cap, o-ring, rear seal and a tube of the "special" lube.
EDIT: I did this repair last fall. Trailer is a 2009 model and I got the repair kit at no charge - wheel hub was still under warranty. Check warranty on yours.