We have been using Avery Signflex for curtain over the past 15years and found when its down its down and as Robert says can be a nightmare to come off.
Not used APA as far as i’m aware so cant comment on that.
Lately we’ve been using inks to do curtains tho.
From what i hear, new curtains are coming with a coating on them that protects them from weathering quicker and like cheryl says, if this is not cleaned off with any kind of solvent then you may have a better chance of getting the vinyl off after a short period.
All i can say is best wishes removing vinyl from a curtain