How do you get outdoor LED strips to stay on vinyl soffits when the adhesive just peels off?

The trim piece at the roof edge is ussually your best bet, and here's why. On most houses the fascia is just a thin aluminum wrap over a wooden board behind it, so it's hollow on top but solid underneath where it matters. I stopped fighting the soffit entirely and went with clips that hook over the bottom lip of the fascia instead. The vinyl soffit panels have nothing behind them, like you found out, but that fascia lip is a ready made rail if you look at it from below. I used the kind of clear plastic clips with an open slot that slide up over the lip and pinch the strip in place, no adhesive touching anything. Spaced them about a foot apart and the whole run has survived two winters now in Minnesota with wind and ice, whereas my first attempt with the factory tape lasted about three weeks before the heat lifted it. One thing to check before buying anything though. Get up on a ladder and feel under the fascia edge. Some houses have a drip edge or gutter apron right there that gets in the way, and occasionally the fascia sits flush with nothing to grab. If the lip is there, those clips will be your answer. On the corners, spacing the clips closer is exactly right. I put one right at the turn and one about four to six inches on either side of it. The trick is not trying to bend the strip sharply, which kinks the solder joints eventually. Let it make the turn over a short distance with those tight clip points holding the curve, and don't pull the strip tight when you're placing it. Leave a little slack so the corner isn't under tension when the strip expands on a hot afternoon.

If your fascia lip doesn't work out, the fallback people use is binder clips hooked over the lip with the strip caught in the metal handle loop. Ugly up close but invisible from the yard, and cheap enough to test before committing.

Following this because I did pretty much the same thing last summer, stuck mine up with the tape it came with and watched it peel off the vinyl like a banana skin by August. The clip idea makes sense but I can't picture how they work. Do they hold the light strip by squeezing it in a groove, or does the strip just sit loose in there and the clip only stops it swinging? I keep imagining mine sagging between the clips on a warm day.

One more thing to consider if the fascia lip is a no-go. Mine runs along the return side of the house where there's a gutter, and I just clipped the strip to the outer lip of the gutter itself. Same no-drill deal, and gutters never move. Corners were the annoying part for me too, mine wanted to twist instead of bunch. Rolling the strip 90 degrees as it turns so the flat face follows the roofline fixed that, took a couple tries to get right.

Zip ties ended up being my answer on a similar setup. The vinyl panels on my porch have those vent slots running along them, and I found I coud thread a tie through two slots and loop it around the strip without any adhesive or holes. Used the UV rated black ones from the hardware store, the cheap white ones went brittle on me the first summer. Only catch is picking slots that still leave most of the vent open so the attic can breathe. Mine went through a winter and a couple of windstorms without a single tie letting go.

I've hung maybe 30 of these runs on customer houses and the thing folks always skip is planning for removal. Cheap strips die section by section, so leave a service loop tucked up near the gutter at one end, long enough that you can unclip the whole run and lay it on the lawn to find the dead spot instead of working off a ladder. Also mount it in the morning while everything's still cool. A strip measured and clipped at 85 degrees will sag like crazy once it cools down that night, and you'll spend the summer chasing it.