I'm not sure how you guys do vacs in the states but we use these for all our cvac rough-ins [Link: broan.ca]

SB, a couple options:
1. I would cut the wall mount portion off the above plate and get exact measurements of the unit and cut that out using plywood (ensuring the plywood is the proper depth of the bracket). Place a coupler on the end of your pipe (Don't Glue it on) and cut a hole in the plywood the diameter of the coupler. Secure the plywood to the form drilling through the plywood first. then place your pipe into the hole you cutout. Once the concrete has been poured and the forms removed you will be able to slide that bracket onto the pipe and then secure the inlet cover onto the bracket. This is providing you are wanting a cover plate sitting flat against the concrete.

2. If wanting the face of the cover plate to be flush with the face of the concrete, you will have to do the same as above, however cutting out the plywood to be the size of the inlet plate. You will most likely also have to make the depth of recess in the concrete deeper to allow for the pre-construction bracket and cover plate. Additionally you could screw option 1 to option 2 and that will get your final depth.

You don't have to worry about the form being "damaged" as the cover plate will hide any imperfections in the concrete in this area.

Also you could as the company doing the concrete if they have an old form you could use to cut up that would allow for a cleaner finish. The concrete company just has to ensure that they aren't sparingly with the oil on your plywood to ensure it doesn't stick to the concrete and leave pieces of wood.