I have a character in my story who is both a vampire and a practicing Muslim. I have another who is also a vampire and she subsists off menstrual blood. [Nutritious :>] However, I imagine Sam (the vampire I mentioned first) would have some objections to this, and it is also a tad weird. Asking your friends to donate their period blood is a lot weirder than asking your girlfriend. However, I´ve heard that things like coconut water can be used for blood transfusions. So can vampires live off those as well? In addition to or instead of blood? Secondary question, if vampires are well-known, would the practice of feeding off period blood be accepted or kind of ignored?
As several commenters have pointed out, vampires vary a lot between different works so it's really up to you.
Coconut water doesn't work as a whole blood substitute because it lacks blood cells. There is an urban legend about it being used as a blood plasma substitute, but that is largely a myth. Coconut water has been used intravenously as a way to hydrate a patient in an emergency. In those cases it was an isotonic saline substitute rather than a plasma substitute. It is usually sterile until the coconut is cracked, and contains useful minerals and carbohydrates, but is not very similar to blood plasma. So if your vampires need blood partly for nutritional purposes, they're out of luck.
On the plus side, coconut water has to be extracted from a relatively fresh coconut, so you could argue that it somehow contains the life force of the coconut, in the same way that blood is said to contain peoples' life force in many vampire stories. You'd need a lot of handwavium to explain why other types of seed wouldn't work - especially if fresh farm animal blood isn't good enough either (which seems likely if one of you vampires needs to use menstrual blood).
That really depends on how vampires work in your universe.
Every worldbuilder makes up their own rules for vampires in their world, depending on what is required for the stories they want to tell. Explanations for vampirism in fiction vary from "it's magic" (which means you can make up pretty much any rules you want as long as they are internally consistent) to various scientific explanations of varying degree of detail and plausibility. There is certainly precedent for vampire stories exploring "what if vampires could survive on something else than blood" (like True Blood). But there are even more vampire stories where having no other options than to drink fresh human blood or perish is an integral part of being a vampire (classic Dracula vampires). So don't feel pressured to get your vampires "right". Create the vampires you need for your world.
So the real question is: Do you want vampires in your world to be able to survive on coconut water? Would that enable a certain plot you have in mind? Then do it. Or would that ruin a certain plot you have in mind? Then don't do it.
Or as TVTropes writes: Our Vampires Are Different.