I have a very limited diet and recently got systemmic spontaneous tendon tears and pain where the muscle ataches to bone (enthesitis). I am also on long term steroids for a spinal issue, but I think vitamins have a role. Any clue?

Well, there is a cause that is a drug. The fluoroquinolone family of antibiotics cause tendon ruptures. That would be Cipro, Levaquin, Avelox as examples. The mechanism for this still not understood, and it can occur months after using the antibiotics.
(can be very delayed in appearance).

Tendons contain silica, and supplements with silica (or horsetail extract) are typically used for tendon issues. Many veggies and fruits have silica in them, like bananas, whole grains, green beans etc.

Often people with bone issues may be low in Vit D and magnesium. Other cofactors are B12 and vitamin K2 (from green leafy veggies).

Steroids are known disrupters of bone metabolism. I'd get a B12 and Vit D test ASAP. Your D target should be 50ng/ml US units. Don't use the RX version given by doctors to raise levels, since that is D2 and doesn't work well. The average dosing is 1000IU D3 for every 10 ng needed to raise.

Your B12 should be at least 400... don't accept "normal" from your test, since US ranges go down to 200 and that is too low for health, we know today.