"Mundane" ventriloquism is very much an act of misdirection like most parlor tricks. You "distract" the audience by moving the mouth of the puppet while they see your own mouth isn't significantly moving. To accomplish this, the timing of the "lip flaps" of the puppet must be very well in time with your speaking. I'm not entirely sure the bond between a summoner and eidolon will encompass such fine motor control coordination.
Now, that having been said, here's how I'd go about implementing it; keep in mind, this is entirely houserule/GM fiat territory. It should be an opposed check by the listeners' Perception or Sense Motive vs your Perform(Act) skill. Your Eidolon is required to Aid Another in you making this check (also Perform(Act)), except that instead of giving you a bonus if he succeeds, it imposes a penalty if you fail; your Eidolon's mouth-flaps weren't up to par so he made the whole attempt less convincing. One check should be suitable for 1 minute of conversation.