I am finishing a basement bathroom and preparing to lay down vinyl flooring. Before I do that, I wanted to check how my toilet flange would fit sitting on top of the finished floor, and I noticed that the concrete hole is so large that I will not be able to screw into anything under the finished floor when I put it in. What should I do? Should I try to find some material to fill in the space? look at something to put under the flange to keep it stable? I'm guessing that I shouldn't just screw it into the vinyl click-lock flooring I'm going to use for the finished floor...
Pour concrete (or mortar mix) to fill the space and embed your bolts into.
Wrap or otherwise protect the pipe. The UPC says:
312.1 ...Voids around piping passing through concrete floors on the ground shall be sealed.
312.2 Piping in connection with a plumbing system shall be so installed that piping or connections will not be subject to undue strains or stresses, and provisions shall be made for expansion, contraction, and structural settlement. No plumbing piping shall be directly embedded in concrete or masonry....
312.10 Sleeves. Sleeves shall be provided to protect piping through concrete and masonry walls and concrete floors....
You can fill with concrete or sanded cement, and in our jurisdiction it can be in contact with the pipe (see comment by @ecnerwal)
It's common to leave space around a shower drain pipe in a concrete floor, so that there is some movement in the pipe to adjust for the precise alignment with the shower base drain hole.
Also, a flange that's glued to the pipe, which in turn is cemented in, does not have to be bolted. My plumber did not bolt mine, and it's 100% firm after years.
It looks like your flange is a 3' internal model (goes into the pipe). This means you can fill the hole around the pipe almost to the top without having an issue with the glue joint. Tape around the pipe with some heavy duty duck tape and fill the hole with Sand mix(no stone). The tape is in case the flange breaks and has to be changed at a later date. The outer pipe surface will remain fairly clean. After that install your flooring and glue your flange down to the floor. Flange should be resting on the new flooring. With a slab floor, I always like to seal around that connection with a little silicone. No other bolts but the toilet flange bolts -aka-Johni bolts required.