Hi, Paul, welcome to the discussion. It's always good to have fresh input, hopefully it stops us "regulars" from getting stuck in ruts of our own making.
I think you have raised a couple of interesting points.
1. As you rightly point out: "A vacuum has physical dimensions. Absolute nothing won't have any dimensions at all." In other words, a vacuum is something (although that may not have been realised before the advent of quantum theory), while nothing is, quite simply, nothing.
2. Is there a difference between nothing and something that is infinitely small? which is a paraphrase of the OP.
I suspect that this is something that could be debated for hours without reaching a point where everyone necessarily agreed. This would tend to suggest that, either, it is a matter of opinion, or that more work needs to be done on the concept of infinity, as distinct from mathematical infinities.
My own opinion is that "infinitely small" and infinitely large" are concepts that arise out of our necessarily finite understanding of infinity. Neither has any real validity in terms physical infinity, but both may be valid in terms of our attempts to explain specific points.
Does that make sense? If not, that's probably because I have not expressed it very clearly.
