Don't know where you are located. I'm in metro Atlanta and did my underdeck 2 or 3 years ago.
I would not recommend vinyl. You can't put lighting, fans or anything else in it. If one section goes bad on you down the road, it looks like piss when you put a non-matching piece in. Plus, without lights, a space is only semi functional.
I need to get my pics up on a blog or somewhere on this forum. (been sayin' this for years now
I used Magnolia underdeck product made locally here. They have installation crews but will sell and deliver product to you - which is what I did.
I have had ZERO issues with the Magnolia system (NO LEAKS or any blockage). The design is superior and the level of support and patience they have with first timers like me was off the charts.
Magnolia uses a system of interlocking finished aluminum U-channels supported around the perimeter and sloped to hidden gutter-type drains. I'm no great shakes at DIY anything but was able to install five can-lights, a ceiling fan, hidden cove lighting beneath the underdeck system, and a track from which outdoor screen "drapes" are used to keep out bugs & skeeters. Also put in a couple outdoor speakers, stained the concrete floor and The Bride picked some nice furniture for our outdoor living room. This and my home theater system DIY are the best things I've ever put together.
PM me if you are still considering options. I'll send some pics.