I agree with ginger899's comments. I would try to avoid doing anything that would stress the dog, and I think surgery to remove a lipoma would probably be very stressful.
My last dog, when she died at age 14, had a bump or lesion -- I don't know what to call it -- on her foreleg that I think was probably an embedded tick head or something like that. She had it for the last several months, at least, of her life - maybe more like the last year. It was something that, if she had been younger and in good health (she was chronically ill by that point), I would have had removed. The vet could have done outpatient surgery on it with just light sedation and a local anaesthetic. She would have been sent home with, probably, two or three stitches at the site. But by that point in her life, I felt that having the surgery would have bothered her more than leaving the lesion alone did.
Congratulations on taking such good care of your friend that she has been with you for 18 years. That is rare to be able to have the pleasure of a dog's company for that long.
Fatty tumors, or Benign Lipomas are actually quite harmless, so long as they do not put pressure on surrounding nerves etc. They might look gross, but if this tumor appears to be causing her no problems, -if she is her 'normal' self in spite of it, and it isn't interfering with her doing things -then my advice would be to leave well alone.
She will have adapted to the different shape of her body, and so long as she shows no symptoms of pain, I do feel this will be safe to leave.
What does your vet suggest?
At 18, your dog seems to be doing well, considering she has blindness and deafness. She will probably continue that way. I wouldn't upset the apple cart for this Lipoma. If it were a cancerous tumor, that would be different of course.