>>I've often noticed in south Indian vishnu temples of a deity quite
similar to the elephant god/ganesh/vinayak with a "namam " . Is >>this lord
ganesh /some other deity?

I believe the deity is actually Lord Visvaksena, the Chief of (Defense!)
Staff, for Sriman Narayana's services at Sri Vaikuntam - His permanent
abode.

According to our philosophy Sri Visvaksena is our first and foremost
Acharya - since he was taught directly by the Divine Couple. Visvaksenar
subsequently passed on the knowledge to our other Acharyas - beginning with
Sri Nammalvar. We regard Sri Visvaksena to be one among the nitya-sooris or
divine souls (?), along with Sri Adisesha, Sri Garuda, Sri Sudarsana, and
Sri Anjaneya.

Please refer to Bhakti list's archives, for further information.

Regards,
Sriram