ah, a wonderful question and so fitting given my current obsession, which colours my days dark and my dreams scarlet red.
let’s abandon all and any popular and modern ideas of vampires, even if they might be dramatic enough to tempt us.
there are three beings that ought to be mentioned: strzyga (or strzygoń, strzyż in the masculine form – but while strzyga’s name comes from Latin and Greek, the male form is purely Slavic, both linguistically and culturally) then upiór/upyr, and then wąpierz/wampir.
depending on the region, as it sweetly is with Slavic lore and myths, you’d either hear they’re the same thing – the names upyr and wampyr seem to really dictate it might be just a slight difference in naming the same creature – and in other places you’d hear that no, good gods, these are three different creatures, all with their own characteristics, preffered ways of tormenting mortals, and ways of killing them (again. at last. finally.)
in some places they know only one, or two.
the differences are sometimes that a strzyga you can be while you’re still alive, while a vampire you become after you die, and upiory are less physical and more like spirits… but very often these differences don’t exist. depends. oh woe.
another crucial thing is remembering that we Slavs all have the same roots and the same fears, but they manifest differently, and there will always be differences between how we see some demons and creatures here in Silesia or Czechia, or Ukraine, or somewhere near the Baltic sea, or far down south where Balkans spread and rule. for me here upiór might be one thing, and strzyga another, and then someone in Russia or Slovakia might hear the same thing and correct me, outraged.
and we’d both be right, such is the glory of Slavic beliefs and folklore.
so, instead of differences – you have to forgive me – I’d rather focus on similarities, as again, they might be all the same or separate things altogether. who knows…
for me, personally, the border between strzygi, upiory and wampiry is thin, wonderfully thin.
the first thing would be signs of being one of those while you’re still alive – this is especially prevalent in strzyga lore, given the belief that you can be a strzyga while still alive and only after death the “evil” part of your soul (or the evil second soul…) reigns and wreaks havoc among the living. however, many of these signs would mean you can be a vampire. again, same thing for some, separate for others.
children born with teeth or who grow two rows of teeth; strange marks or scars on body; too much hair, being born with full hair or growing it later, bushy eyebrows, hairy hands; strange eyes; big head (yes); malformed body parts or being physically disabled (hunch back, malformed fingers, one leg shorter etc etc) or being born with your heart in the right side of your body (accompanying the belief that strzygi have two hearts/two souls)
your chances of being – or becoming – a strzyga/wąpierz/upyr were greater if you were a witch or practiced magic (so this also means shepherds, as they would always dabble in folk medicine and charms), if you died tragically or suddenly (terrible illness, suicide, childbirth), if you died without being baptised (thanks Christians for adding to the criteria), if you were murdered, if you went mad, etc etc… but people who died peacefully, of old age, in their beds, surrounded by loving family, also could become those terrifying things, so don’t worry, there’s hope for everyone.
now, signs after death: pink skin, red lips (as from drinking blood), open eyes, different position than how the body was laid to rest initially, somehow escaping the seemingly inescapable wonders of rot and putrefaction, scratched/broken/split coffin, as well as finding pieces of strange things in the deceased mouth (so hungry they devour whatever! usually what is meant by that are pieces of fabric that the body was covered with that somehow found their way into the oral cavity)
and then, of course, what they surely have in common is being malicious and tormenting the mortals, being violent and having a strong affinity for blood (although, surprisingly, that is not always the case, nor was it always the most defining and characteristic thing) and the ability to shapeshift – usually into moths/bats/crows, but also dogs, cats, owls… and cows, apparently. cute.
and a stake through the heart to kill them! oak, aspen, hawthorn – or something made from iron, even.
although, as folklore sweetly reminds, the stake, most popular and reliable method that we all know of… rarely works, really. some strzygi and wampiry are simply too spooky and too strong to be discouraged by a stick.
so burn the body, preferably.
if anyone has their own things to add, please do – and again, we’re all right in those beliefs, no matter how different or even contradictory they are.
ah, Slavs and their sweet undead.