The real risk isn't the vinyl itself
Vinyl panels overlap, with small gaps designed to let the material expand and contract with temperature. Water at high pressure, aimed upward or at the wrong angle, gets driven through those gaps and behind the siding.
Once water is behind vinyl it doesn't drain well and doesn't dry quickly. Over time that means mold growth in the wall cavity and, in the worse cases, damage to sheathing and insulation. None of it is visible from outside, which is why it's often discovered years later during a renovation.
What high pressure can do to the vinyl
Older vinyl becomes brittle with UV exposure over the years, and brittle panels can crack under direct high-pressure impact. Aged panels also develop a chalky oxidized surface, and aggressive pressure can leave that oxidation looking blotchy and uneven rather than uniformly faded.
The method that works instead
Soft washing. Low pressure with a cleaning solution that breaks down the algae and mildew chemically, applied top-down, then rinsed. It removes organic growth more thoroughly than pressure does — because it kills the root structure rather than knocking off the visible layer — and it doesn't force anything behind the panels.
If you're doing it yourself
Keep the pressure low, stay under about 1,500 PSI with a wide fan tip, never aim upward under the panel edges, and work top-down. Wet the landscaping before you start and rinse it afterward. And be realistic about ladder work on a two-story home — most of the injuries in this line of work come from ladders, not equipment.