Can the Hoover SteamVac be used just as a vacuum?

No. The SteamVac has a tank for catching dirty water extracted from the carpet, but it doesn't have a filter bag or canister like a regular vacuum. When used dry, whatever you suck up would get blown out again. Furthermore, the vacuum opening on the SteamVac is a long, narrow slit optimized for maximum suction when extracting water, but not effective for objects larger than the width of the slit.