Hi Nick,
A really interesting weight indeed. I have seen this feather effect on other Ysart pieces (but I was not sure whether they were Salvador, Paul or another brother). Having said that this feather effect is also common in weights from other countries (e.g. Belgium).
The lampwork is sweet but I have no idea who could have made it (except that it is not by Paul Ysart). It may be Salvador's but I have seen too few of his lampwork pieces to hazard a guess.
Yes, if this weight were Vasart it would be quite rare indeed.
I am sure that the Ysart specialists will tell us more though.
SophieB