Q: After communion, the vessels are purified either at the altar, the credence table, or the sacristy. Is there a preference?
A: The answer is in GIRM 279:
“The sacred vessels are purified by the Priest, the Deacon, or an instituted acolyte after Communion or after Mass, in so far as possible at the credence table.”
The altar is the first option listed in the circumstance when the priest has no deacon (GIRM 163). Even then, I usually go to the credence table to perform the action.