If you mean a vitamin that supports healthy blood vessels and circulation, vitamin B3 (niacin) can cause blood-vessel dilation and flushing, but it is not recommended to take niacin specifically to improve circulation without medical advice. High doses can cause liver damage and other side effects.
For general vascular health:
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Vitamin B12 and folate help maintain normal blood cells and homocysteine metabolism when you’re deficient.
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Vitamin C supports collagen and blood-vessel structure.
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Adequate vitamin D is also important for overall health, but supplementation doesn’t reliably “open” blood vessels.
If you’re asking because of your nighttime leg cramps, vitamins aren’t usually the first thing to take—the cause should be identified, especially if you also have cold feet, numbness, swelling, or pain when walking.
