Forget about glue - I see many items where glue has been tried
before they come to me.
The solder used originally will likely be soft solder which was
plated over. Try scraping some of the remains of solder at the break
and see if it is soft or hard solder.
The vase can be repaired using either silver solder, or a lead-free
soft solder that is 97% tin. The tin solder will not tarnish and is a
close colour match to silver. The crucial points with silver plate
aRe: there is no leeway to do any cleaning up afterwards, and if lead
solder was previously used then it must be completely removed from
the broken joint.
To join using medium silver solder - make sure there are no other
joins where lead solder was used. Sometimes cast pewter
embellishments are used. If either are suspected then go to lead-free
soft solder. Medium silver solder is better than easy because it is
less likely to go wild and spread under the plating where it is
impossible to clean up, but easy silver solder can be used to sweat
with careful heating. Cover the whole item in borassic acid or a
proprietry coating. Set it up so that the parts will align perfectly
when the solder melts - with silver solder there may be only one
second chance. The parts can be wired, or propped up, and a
turntable is essential. Flux and solder. Pickle gently and clean up
with a brass brush and lubricant (I use bi-carb soda). If there are
any blisters in the plating the brass brush will compress them.
Polish with Silvo followed by a silver cloth.
To join using lead free soft solder - Same as above but no need for
the fire coating. Flux with ‘killed spirits’ aka ‘zinc chloride’ and
generously tin both surfaces with a soldering iron. Set it up, and
then gently heat the item until the solder sweats and there is a nice
meniscus. It is possible to make adjustments by remelting the solder
quite a few times, but watch where the heat is going so as not to
draw solder over the surface, soft solder will run wild over the
surface towards the hottest area. Clean up as above, but no need for
pickle, and bi-carb of soda is necessary to neutralize the flux.