The body has a set volume of blood, so when someone is given nitrates to vasodilate (primarily) their veins, would this not increase the total volume of the vessels in the body? If this is the case, I feel like this would mean there wouldn't be sufficient blood to fill all vessel in the body? So I am assuming there must be vasoconstriction somewhere else in the body to balance this out, such that the total volume of all vessels remains unchanged. I feel like there is something I am not thinking about here, any help would be appreciated!
You can think of blood volume and blood vessels as a closed system for your nitrates example.
Vasodilation does not change blood volume; it changes the volume the container (the blood vessels) can hold. The result is that with more "room" for the blood to occupy, your blood pressure falls.
That's why hypertensive emergencies (e.g. hypertensive encephalopathy) were treated with IV nitrates (now a host of other drugs as well), and why patients new to nitrates are told to sit when they take a dose of sublingual NTG (there may be enough relaxation that the relative hypotension causes lightheadedness.)
However, it's a dance; baroreceptors will sense the decrease and, yes, some vessels will respond with vasoconstriction.