He may have intestinal parasites from eating the mice. Giardia for example which you can read about here

http://marvistavet.com/html/rat_poison.html

If any of the mice might have gotten into rodent bait before being caught he could have ingested some of that with the mice. You can read about that here

http://marvistavet.com/html/rat_poison.html

Mice can also carry leptospirosis which you can read about here

http://www.peteducation.com/article.cfm?c=2+1556&aid=454

If his stool seems dark/black and his vomit has a coffee grounds look to it or you see blood in the vomit that might be a sign of rodent bait.

But it could also be a condition called Canine Hemorrhagic Gastroenteritis (HGE) which you can read about here
http://www.sniksnak.com/doghealth/hge.html

Another possibility is, especially if he had a fatty treat before this started, pancreatic inflammation which you can read about here
http://www.peteducation.com/article.cfm?cls=2&cat=1580&articleid=335

If your dog might have gotten into anything toxic (including toxic plants), appears dehydrated (see if a pinch of skin over the shoulder blade stays stuck in a tent form or flows back down normally when released), has a rectal temp over 103.5F (normal is 100.5-102.5 Fahrenheit or 38 - 39.2 Celsius ), has gums or tongue that look white, blue, gray or yellow tinted where they are normally pink, has a slow color refill if you press on a pink area of gum or tongue, is having difficulty breathing, is vomiting or passing blood or black stool, or is becoming more lethargic and non responsive to you as time goes on then you may want to get the dog in to see an emergency vet if one is available in your area.

This site may help you locate an emergency vet if you need one

http://www.vetlocator.com/hotline.php

If he doesn't seem that bad an over the counter med you might try for digestive upset would be Pepcid or Tagamet or Zantac.
Be sure to tell the vet when you see them if you use any of these.
You can read about dose amounts (and cautions) scrolling down on the pages here
http://www.petplace.com/drug-library/cimetidine-hcl-tagamet/page1.aspx
http://www.petplace.com/drug-library/famotidine-pepcid/page1.aspx
http://www.petplace.com/drug-library/ranitidine-hcl-zantac/page1.aspx
If that helps then you can see if some bland food and plain pedialyte added to the water bowl will help.
I would suggest trying boiled chicken meat or hamburger with the fat drained off, mixed with really well cooked rice in a ratio of 1/3 meat to 2/3 rice to see if that works. It's not a balanced diet but OK to use for a few meals while working on controlling digestive upset.
But if he is vomiting a lot, then if you can see a vet with him today or tomorrow you should try to do so.

Hope this helps you!

Thats probably his yorkie side of the family - they can be great mousers!

Thanks for the above accept!