I know the first S-3 Vikings came into service just before the Vietnam War ended but I was wondering would any have seen service over Vietnam?

From Wikipedia:

On 20 February 1974, the S-3A officially became operational with the Air Antisubmarine Squadron FORTY-ONE (VS-41), the "Shamrocks," at NAS North Island, California, which served as the initial S-3 Fleet Replacement Squadron (FRS) for both the Atlantic and Pacific Fleets until a separate Atlantic Fleet FRS, VS-27, was established in the 1980s. The first operational cruise of the S-3A took place in 1975 with the VS-21 "Fighting Redtails" aboard USS John F. Kennedy.

So, to answer your question, it didn't see service in Vietnam. Its first operational cruise didn't take place until 1975. The Grumman S-2 Tracker was deployed in the SEA area of operation, though.