Your frustration is well understood by many. LP's and VEP's are not necessarily the "holy grail" of diagnosis. I have always had a normal LP. My VEP was abnormal, but I attribute this to my right eye problems and my cognitive concerns.
As to "what else could it be"..... there are over 400 possibilities. Now, many of these could be easily discounted, but all must be discounted to be left with MS.
Your doctor may be a fence sitter. They do not want to make the decision. In those cases, tell them straight up to get off the fence because you need to be able to live your life. You might even seek another opinion because there are indeed doctors who will get straight to the diagnosis without any hand wringing. In the meanwhile, one day at a time. I will keep you in my prayers.