My 979, which is probably of a similar era based on the "regular" (non-grub) seatpost bolt, has the size stamped into, as I recall, the headtube lug on the non-drive side.

From what I've seen, small frames like that (in general, not necessarily Vitus-specific) seem to hit a top tube limit of about 51 cm, so I guess 50.5 could theoretically be possible.

This catalogue page (PDF) shows the smallest frames having a 53 cm top tube, but a 48 cm model is not listed. Perhaps a 48 was added later with a shorter top tube?