I'll try to help. How long has she been on Lasix? Is she on any other drugs? Has she had a cardiac work-up? How old is she? Is she urinating in the house?
To be honest, in my opinion, your dog really needs a further work-up. Did they do bloodwork at all? Chest xrays?
If her coughing is improving with Lasix, then you can let your vet know that. However, if she isn't taking it at all, then you won't know that. However, I wouldn't personally start a dog on Lasix without doing more of a work-up first, because we don't know what else may be going on with her. Also, I like to know whether kidney function is normal first and whether the pet is dehydrated, etc, before Lasix. And whether or not the vomiting of phlegm is related to the coughing or another problem, I don't know.
Also, Lasix is a diuretic, so if anything, it'll make her urinate more. Make sure she is drinking plenty of water. If she is holding her urine, try carrying her outside to her usual spot and see if she'll go there.
As far as throwing up her food, try feeding her very small amounts at a time more frequently than usual. Try a bland (1/3 chicken boiled 2/3 boiled white rice) diet for a couple of days. Also, don't let her guzzle a ton of water at one time. Small frequent amounts will stay down better.
Hope this helps and good luck!
If she isn't drinking anything by tomorrow, I would seriously consider having her seen at the Vet ER. Especially on Lasix, she needs water. Try giving her some ice cubes, some dogs will lick those. Also, measure the amount of water you put in her bowl and then measure what is left in 12 and 24 hrs. That way, you'll know exactly how much she is drinking or not.
Keep us posted!