2015 Ford F-150 - Trans fluid

In my 2015 Ford f-150 manual with 3.5L ecoboost states to use Mercon VL from Motorcraft to replace the transmission fluid. I had my trans fluid changed and found out a different fluid was used. I don’t know the brand as I want to have it replace with Mercon. 1. Should I be concerned with running with a different brand? 2. Mechanics don’t recommend doing a flush in this transmission 6R80. But when having the trans fluid drained and adding Mercon, will this blend with the existing fluid and cause other issues? 3. Or contact Ford?

Your truck uses Mercon LV, not VL. Unless the fluid used specifically states it can replace Mercon LV, it can’t. There are fluids compatible with Mercon LV. Valvoline makes one and it is in my trans. That is OK.

If an incompatible fluid was used, if it were my truck, I’d drain and refill, twice, to get whatever they put in back out.

Have you asked if the fluid was an approved fluid ? That seems like the first step . And if the answer still not satisfy the web site of the fluid brand night show what it can replace.
Or just go to the ford dealer near you and say you want the fluid replaced with the correct product.