Perhaps it's been awhile since you've read Adam Webb's book "Dumb Angel"...or my essay on the POB/Bambu sessionss at
www.beachboysarchives.com, Mikey...if you go there & look up "Only With You" on the PDF, you'll see why...

"Although the track for this version was cut in September '76, final vocals were apparently not added until September '77. In Adam Webb's book, British rock journalist Roy Carr recalls accompanying Dean Torrence on a trip to Brother Studio a few weeks after POB's release (Dean was delivering promotional photos to Dennis). The two of them were soon enlisted by Dennis to sing some backup vocals on this tune, which Carr was familiar with from Holland. Roy says "The whole backing was done but he said he wanted to do a layered vocal on it. So he did a guide vocal himself in about fifteen minutes, doing two or three takes, and we spent the whole afternoon just layering this song and another one I can't remember the name of".