varPath = "path" & VendName & "" & InvNumb & ".bmp"
Where is "path" coming from and what does it look like? If it doesn't end in a slash, you either have to edit that source so that it does, or concatenate it into your constructed path string. I doubt it should have quotes around it as well. Also, InvNumb looks like an incomplete reference. Think of it as telling someone to meet you at Main Street. It might work sometimes, but it's flaky.
Path is a reserved word and shouldn't be used in this way. This probably should look more like
varPath = strPath & "" & Me.txtVendName & "" & Me.txtInvNumb & ".bmp" where (e.g.) txtVendName is the control on your form which contains part of the path string.
You'd use Application.FollowHyperlink, but there are other (and some would say better) ways to open files. You might get security warnings using this method, which was (I think) originally designed to open web pages. If your file path may contain characters which would be problematic for this method, you should check out this code:
http://allenbrowne.com/func-GoHyperlink.html
The more we hear silence, the more we begin to think about our value in this universe.
Paraphrase of Professor Brian Cox.