Spoil your dog as much as possible, try to get her to eat anything, but most of all find out exactly what is wrong with her. If you don't, you will always wonder if you handled the situation right. Knowing exactly what is wrong with her and what can or can't be done will give you some peace of mind if she has to be put to sleep, because you will know you've done everything you can. You and her both deserve a definate answer from your vet of what is wrong with her.
Oh sweetie, I am so sorry. Even if it isn't a tumor, your dear friend is desperately ill and suffering. There are so many things that wear out as dogs age, and it's not always easy to figure out exactly what is going on. Has your vet just not been able to work in an x-ray yet, or has he given you a reason for not doing it? If he is confident in his diagnosis of some kind of cancer or other terminal condition, then you need to have a serious talk with him so you understand exactly what is going on and what will come in the next few days. From what you write, I think you are very close to having to say goodbye, and it breaks my heart to say that.
I've been through this exact situation several times in my life, and completely understand how emotionally traumatizing it is to be faced with a euthanasia decision. I have finally learned that it truly is a gift that we give to our fur-kids as a release from suffering. That knowledge eases the sting at least a little bit.
The way I reach the decision to euthanize is to think of three things that bring the most joy to my dog's life. As my dog loses interest or cannot do those things, I know the time has come. Pain, nausea, vomiting and diarrhea can be fairly well-treated depending on the underlying condition, but I also have to consider general quality of life even if those problems are taken care of. Each dog is different, and you know your girl better than anyone else, so ultimately it is up to you. It's a terrible decision to face and it's so difficult to maintain objectivity with emotions running amok.
For now, is your vet able to offer anything for the nausea and vomiting? Cerenia seems to be the gold standard for dog nausea these days, but it's awfully expensive because it's fairly new. Good old cheap phenergan worked wonders for my dog who died of kidney failure, which brings on terrible bouts of vomiting. Believe it or not, thorazine can also used for dogs with excessive nausea and vomiting in dogs, and it's a very old and inexpensive medication.
Does your vet think your girl is in pain? It's terribly hard to assess pain in dogs because most of them are very stoic about it. People can communicate pain by speaking, yelling, crying, etc., but most dogs seem to just accept it without complaint. My first dog died of pancreatitis which is horribly painful, and he never made a sound, even on palpation of his abdomen. The vet said he'd never seen such a stoic dog with pancreatitis.
I guess all this boils down to is making your baby as comfortable as possible for as long as possible while you make your decision. There is no right or wrong answer here as long as comfort measures are taken. God bless you, and please stay in touch. We're all here for you. :-)
Ive just found out that the vet doesnt even know if it is a tumor or not. She hasnt had an X Ray or anything so im making sure he does check her out properly and see what is wrong. She doesnt seem to be in any pain, if she was crying and wasnt moving then fair enough, I would allow her to be put down, but im not making that decision untill I know what it exaclly is! Its the least we owe her to know whats wrong before letting her go. She is eating, she just keeps bringing it back up.
Your dog is very old... Seems like she's suffering, maybe it's better for her to go to sleep to stop the suffering. It may be very hard but I'm sure she'll go on a better place. Spend all of your time with her, make her forget the pain and make sure her last days will be her happiest ones :)