First, you need to consider the purpose of a ration balancer- it is to balance out a horse's diet that is primarily comprised of forage (either hay or pasture). Hay/pasture diets are almost always deficient in at least some minerals due to the mineral profile of the soil the grass is grown in. Certain minerals may be deficient in an entire region (like selenium in many areas of the US) or might be specific to that field based on its agricultural history. Unless you do an analysis of your hay/pasture, you can't be entirely sure the nutrient content.
Luckily, most vitamins and minerals have a wide margin of safety, and providing more than the 'recommended' amount for a horse usually isn't an issue. That's why ration balancers & fortified grains work well for a wide range of horses with access to very different forage. There are some exceptions, and some minerals need to be kept in a certain balance with other minerals (you'll often see people talk about the calcium-to-phosphorous ratio, for example) I second the suggestion of FeedXL to make sure you're keeping those minerals in balance.
Ration balancers are low-calorie feeds that include vitamins, minerals, and protein to supplement an otherwise forage-only diet (they can also be used to supplement grain when it's fed below the recommended levels). Most feed companies make at least one ration balancer (some make two- one for grass diets, one for alfalfa diets). You can read up on ration balancers here: Ration Balancer
If you need extra calories beyond a ration balancer, rice bran, beet pulp, and alfalfa (pellets/cubes/hay) are healthy alternatives to feeding grain.
A vitamin/mineral supplement (like SmartVites
) is similar, but does not include protein. Be careful of some vit/min supplements- not all of them provide balanced nutrition (a lesson I learned with my horse!)
I'm not sure what kind of block your vet was referring to, but I have yet to see any block that provides horses with a balanced or complete blend of vitamins & minerals. Horses have not been shown to self-regulate any mineral other than salt. The amount that a horse licks at a mineral block or free choice mineral mix is largely due to the horse's preference for its flavor and/or boredom. Additionally, horses tongues are not rough and it can be difficult for them to get even sufficient salt from a block, much less any other minerals.