Hi Maureen; in answer to your question probably the most complete collected information on the Irish Vances - especially those from Ulster - is Jamie Vans' website at http://homepages.rootsweb.ancestry.com/~vfarch
. I can't guarantee he's got your Vances indexed there already but you've got the best chance of finding them there.
Otherwise if you contacted the Vance family association at http://www.vancefamilyassociation.org
(see contact info there) they might be able to tell you if they have any info on your line or perhaps who might be researching it.
Otherwise you're stuck with Ancestry.com, the LDS Church records (familysearch.org) or direct Irish records (many of which have been indexed by sites like RootsIreland but there is still no substitute for in-country research).
I have some general background on the Vance family name in Ulster in book 1 at http://sites.google.com/site/vancepath
; at least on the most common story of the ancestry of the name in Ireland (the Norman de Vaux). That doesn't mean your Vance ancestry in Ireland might not have come from another origin. But that won't help you if you're looking for specific family lines.
The other usual recommended source is the Vance DNA study group at http://www.vancegenealogy.com
, but they're only researching the Y-SNP DNA lines at the moment (the male lines, following the surname) and so would only be useful really if you have any male relatives who still descend directly from males with the Vance surname and so could be tested (or perhaps have been already) to tell you which of the 9 genetic groups of Vances yours came from (if not a new line!).
Sorry for the general suggestions but best of luck in your search!