How to decrease the saltiness in pork meatballs

To get more flavor out of cumin, you can use whole seeds, and toast them briefly in a pan before grinding. If you don't want to put forth that extra effort, you'll just need to add more cumin. If it's not salty enough, the best solution is to just add more salt (sorry). Salt will enhance the other flavors as well. A bit of cornstarch would help make the spices stick, and dry out the mixture. I'm not sure how much water and vinegar you're adding, but you may just be watering down your beef too much. I usually add just enough to keep the spices from sticking to the pan, but not so much that there is a lot of extra liquid that needs to be reduced.

Your spice mixtures may also contain some form of MSG. You can get a similar umami "punch" by adding a dash of Worcestershire sauce.

Edit: I don't know how I missed this, but the other answers point out the lack of garlic and onion. Your store-bought spice mixtures definitely contain plenty of those in powdered form, which you can use, but fresh is tasty too. Don't add fresh garlic too early, as it can easily burn and turn bitter.

My mother taught me to insert bits of garlic into slits made into the flesh of the roast before searing and then cooking it, something like what's described here.

Some people like cloves in there as well, though I more prefer that with bbq pork. For lamb, it's garlic and green olives. So, accordingly, you should be able to get creative and experiment with injecting some other things so the roast will take on flavors you prefer.