Hey Jim,
I have done this in the past, too. And the vinyl just has no strength for remodeler cans.
To do this, I have used Rough In cans instead.
I disassemble the housings, basically I remove the can from the frame, and the whip to the jbox and set it aside. Then I get the frame into the soffit, and my cordless drill and screw the legs to the framing of the soffit. Reassemble the can in place through the hole. Redo the wiring in the jbox also through the hole. This will also allow for adjusting the can for height because of the adjustable can depth on a Rough In can, so the trim doesn't lift the vinyl soffit.
Here's the hard part. First, you need at least a 5" can, because you need to get your drill, hand and arm in the soffet to screw the frame down.
Second, you will need to cut the frame. So you can get a square frame through a round hole. Cut the frame with a hacksaw directly opposite the attached jbox. Remove the legs/feet. Seperate the frame a little to stick one portion of the frame through, work it around so that it all makes it in the hole. Reassemble legs, set in place, and proceed with mounting.
Here's the REALLY hard part. You have now altered a manufactured item, in a way not stated by the manufacturer. Is this going to void any warranty? I can't answer that. Once it is all reassembled, it acts like it once originally did, as a solid assembly even though there is a cut in the frame.