im guess this is your account? use the login info for this name on the ps3
create a new user on the ps3
then sign in
It seems to me you had an existing PS3 user. You need a new user for each account. So make a new user, then sign in with your new Vita account (just as jon1234 said). If that's not the problem then I'm not sure, but a lot of people come here with that question (i.e. they try can't sign in because their user was associated with another account).