While I can't say for sure (since I don't have a Vista), I would think the panel is slid down onto a few "hangers" and is then held in place by a few screws along the bottom. Feel your way tightly along the bottom of the panel looking for screws that go into the gunwale. Take those out, then carefully pull up on the panel (do not pull outward, just straight up vertically) and it should come up and off. Then you can peel back the vinyl/fabric back which is probably stapled onto the plwood supporting the panel.
Hopefully someone with a Vista that has removed those panels can provide more accurate information. This is just my best-guess.
_________________
1999 180 Horizon OB Fisherman Pkg
San Antonio, TX
Choice of tow vehicle:
'93 Explorer 4x4 (sold)
'05 Grand Cherokee 4x4 w/ 5.7 Hemi (member of the 240k+ mile club!)