Is there a reason to install, say, VLC from F-droid instead of play store? I'm a fan/contributor/user of FOSS, but not an RMS-style purist. Is there some advantage I'd get installing OSMAnd or VLC, etc. from there instead of the playstore? Some privacy thing or frequent updates?

I'm basically worried that I'd be hurting VLC's exposure by not adding to their download numbers in the playstore with no real advantage for me or VLC or FOSS in general. I'm explicitly asking if I know an app is FOSS and ensure the author's the right one, why not download from the playstore?
Given the second para of your question, where you are sure of developer, I would go with downloading from Play Store -:


Faster updates. Updates from F-Droid are typically slower than Play Store, more noticeable if developer updates app very frequently. For instance, check out open source email app "FairEmail" and see the versions on Play Store and F-Droid. Admittedly, this is an extreme example because developer at times pushed out 3 updates per day!
If it is a paid app, it is easier to use Play Store while installing on a new device.
Not sure of this but I have noticed that non Play Store apps are at times not backed up by Google and restored on new devices. Backing up and restoring has too many factors, so wouldn't weigh this heavily.