I didn't understand it completely. I feel there is no AVP like 1* [Vendor-Id]. It may be a syntax to define an AVP multiple times.
This is what ABNF says:
repeat = 1*DIGIT / (*DIGIT "*" *DIGIT)
So it means the vendor-id avp will repeat one more time in message.
Same with the AVP code. if you want to repeat an AVP more than one time you can use the above syntax to do it.
ABNF for diameter