Typically if a abscess forms you take antibiotics,and if it's bad enough they have to also be drained,you can't really "cut out" an abscess,at least not without taking healthy tissue with it.
Your grandmother should receive a pelvic exam to determine the cause and extent of the abscess to determine the correct treatment,a small sample may also be taken to rule out cancer beings she is post menopausal.
I don't know what kind of health care they have in Uganda,but if she is in this much pain this should have already been evaluated and treated with antibiotics and possibly drained.
This is the only thing I could come up with it's not technically "in" the vagina but below the opening,a cyst forms and then can become infected. Here's a link where you can read about this condition,and even if the abscess isn't in this location,it would be evaluated and treated the same.
Thanks for writing to the forum!
A painful boil inside vagina can be an infected Bartholin cyst or an abscess or even an infected polyp. It is difficult to comment beyond this without examining. Since it is causing so much pain and the doctors are not telling you what it is, it would be better to get it removed and then biopsied to know what it is.
Hope this helps. Please let me know if there is any thing else and do keep me posted. Take care!