I am very sorry to hear about your older pup's lethargy, lack of appetite and vomiting today. I am even more concerned that she has not been able to keep water down.
Possible causes of vomiting and decreased appetite include a change in diet, dietary sensitivities or allergies, or eating things that they should not like too many fatty table scraps or garbage, bones etc. Addison's which is a poorly functioning adrenal gland is another possibility for waxing and waning vomiting and nausea. Metabolic organ failures (kidney or liver disease or diabetes), pancreatitis, inflammatory bowel disease or even infiltrative cancers are possible. If she is not spayed a reproductive tract infection (pyometra) is also possible.
If she isn't eating normally and is getting dehydrated because she is vomiting that can lead to lethargy, or it is possible that whatever is leading to stomach upset is making her lethargic too.
Has she eaten anything she should not have recently (toy pieces, bones, garbage)?
Any changes in food or treats?
Ideally at her age she should see her veterinarian for an examination and some testing tonight as well as supportive care like fluids and anti-nausea injections since she cannot keep even water down.
If that isn't possible right away you can give her acid reducers to try and settle her stomach. Either:
1) Pepcid-ac (famotidine) at a dose of one half of a 10mg tablet per 10 to 20 pounds of body weight every 12 hours.
2) Prilosec (omeprazole) at a does of one quarter of a 20mg tablet per 10 to 20 pounds of body weight every 24 hours.
These will reduce stomach acid and could help settle her stomach. These can be used for several days if necessary as they are very safe.
I'd also pick up her food for now. A couple hours after giving one of the acid reducers you can offer more limited amounts of water or ice cubes to lick. She's likely thirsty but we need to settle her stomach first.
No food for 12-24 hours. Small amounts of water only.
After her food fast then start a bland diet of 1/3 boiled, lean hamburger (or boiled, white, skinless chicken) and 2/3 boiled white rice. Give small meals several times a day. Feed the bland diet for several days, then start mixing in her regular diet and slowly convert her back.
If her vomiting or nausea continues then she may need injectable medication from a veterinarian to get her stomach upset under control.
So watch her for continued vomiting even with the acid reducers, blood in her stool or vomit or a fever (more than 103.5F rectally), a tense painful belly or lack of appetite after her food fast. If any of those occur it is time to seek hands on veterinary care.
Please let me know if you have any further questions.