Can you tell me how deep the joist is?
Might measure 7 1/2" or 9 1/2" or 11 1/2"
Seventeen feet seems a might long.
Just to confirm -- The joist is unsupported for a distance of 17'?
For a span that long
most building codes would allow a sag of almost 5/8".
If the vanity is near the middle of the span then having to shim one end 1/2" makes sense.
Since the shower's in place you don't want to jack the joist up -- that would would crack the concrete and limestone.
The simplest repair is to install a 4 x 4 perpendicular to the joists near the center on the joists that're sagging the worst.
Usually make it long enough to support three joists.
Then install a 4 x 4 column to support that.
The column runs from 4 x 4 to concrete floor.
Your concrete floor in the lower level will support the weight.
It's much harder to add joists along side of the sagging ones that run from one end to another. The ends must be shaped so it can be twisted into place right next to the first. Then shims are used on the ends to adjust the height and level.
For a span this long you'd have to add 2 new joists.
Does this make sense?
Have I answered your question?