Well I can't believe I did it . I put 1/4 Plywood down as a underlayment for stick down tile.i just got finished laying the tile and my son says daddy were is the a.c. vent I looked I said omg . Now what do I do the floor is done.. It's a trailer so what do I do go under the trailer? Wow
You should be able to gently tap the area close to where the hole is and listen for the hollow sound that a thin layer of plywood over a void would make. Then using a 1/8 drill bit probe the area to locate the center and cut out a section large enough to give you a look inside. After that gently cut away the rest to the edges of the vent hole.
Find the other side, remove and drill through then make the hole and fit a new cover.
Or consider re-locating the vent if possible.