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When I paint older wood windows I remove the sash and paint it separate. This is allowed to fully dry before being re-installed this technique reduces the chance of the windows becoming "painted" shut.
Since these are new, they may have factory gloss. I would remove that by scuffing lightly with 120 or 150 grit and possibly wiping down with acetone or a deglosser to remove any oils.
The other problem is that if you are using a particularly dark or hot colour, The sun can heat the vinyl and cause expansion problems which are already a concern with vinyl windows, especially
But great amounts of thermal change can stress that bond between vinyl and paint.For dark colours I might be tempted to use 80 grit to provide more 'tooth' to the bond, so there is a good mechanical bond as well as the cchemical bond
Use a paint designed to be used with plastic and use what ever primer that manufacture
Scuff them up with a fine sanding sponge and wipe them off.
Prime them with a PVC primer like Insulx 'Styx', available at fine paint stores. Then you are ready for any good acrylic top coat.
Regular paint will stick directly, even without the prep, but will peel off when scratched or pressure washed.
Also, Vinyl does not last forever. UV rays of the sun will dull and weaken it over time, so paint can have a benefit when properly done. Proper includes prep work like cleaning and sanding, and using the proper primer for the vinyl if one wants the job to last