The image below is from Moncrieff Glassworks company records, it shows Salvador's, Paul and Augustine personal signatures.
I do not claim to be an expert on calligraphy, but there is to my eyes there is a clear likeness between Salvador's signature and the Ysart piping on the vase.
The similarity that I see
1/ the way the "Y" loops at the top and bottom of the "Y"
2/ The space between "s" and "a"
3/ The "r" and "t" look very similar.
This no way confirms it as Salvador signature as it would take someone with knowledge of calligraphy to work that out.
I have emailed Bonhams asking if they can pass on any provenance the have on the piece.