Congratulations on the vise. I'm not sure what you mean by, "The collar that the stem fit's into to attach to the base is very loose and over sized." But mine has a "locking nut" to tighten the collar to the base, and a wing-nut to secure the "stem" into the "collar". So I can make everything as tight or as loose as I want. (not sure why I'd ever want either of them loose though) I assume yours is the same, so I doubt you'll have any "issues" using the base if you decide to go that way. I on the other hand have never used the C-Clamp, but the "collar" hole on it (again with a wing-nut for securing the stem) appears to be the same diameter as the one in the base. So if one bothers you, the other likely will as well. (Though I don't see why either would be an issue.)

---David