Hello I am so sorry to hear that Didi is not feeling well. My name is ***** ***** I would be happy to help.
My concern would be that she is having another flair up of the gastritis or what we call relapsing pancreatitis and that the brown vomit was due to blood. Is it her time I can not really say that without some blood work and an exam. It is very possible if this is a relapse that you can get her back on track with supportive care and medication so that you can have her around for a few more years.
I know it may not seem that she is physically suffering however if she is so weak she is having trouble standing then there is some level of suffering happening.
I have to suggest getting her in to the vet for a hands on exam and at the least some blood work. Once this is done then you can make a more educated decision on whether it is time to let her go. I would not suggest letting this go on for much longer as rarely is a passing due to something like this painless and peaceful and the last thing we want is to let her suffer.
If your vet feels that it is time to let her go they can help you do that without any additional suffering on her part.
I know this is never an easy decision and I do not want you to have any regrets which is something we see often so this is why I suggest having her seen by your vet. If you can get her in tonight that would be best. If that is not possible I would give Pepcid AC 1/4 of a 10mg tablet to help settle her stomach( this can be given every 12 hours) and get her in first thing in the morning.
Hopefully this is something that is fixable and you can spend some more time with her. Please reply back if you have additional questions or concerns and I will be happy to continue. If you have no additional questions please take a few moments to provide a rating so that I am compensated by justanswer for assisting you this evening.