To be considered a vampire a creature needs to meet 3 requirements:
They are no longer alive, but remain conscious.
They require outside sustenance to survive.
They inherited at least one vulnerability from a sire.
This vague collection is intentionally non-specific. The most commonly considered vampires are the ones who feed on blood and are vulnerable to daylight. Based on the power of belief, some vampires are vulnerable to holy water and religious paraphernalia.
Like any species, blood sucking vampires have cousins and distant relatives who feed off many different things after their death.
Likewise the vulnerabilities vary. Sunlight is the most common vulnerability, but others include silver, moving water, and wood.
In general a vampire can always be defeated by chopping off their head, but most vampires these days are relatively harmless. If properly managed a vampire can go their entire existence without killing anyone - usually.
In the 1200's-1600's the Sentinels were at their peak. During the heights of their power the hunted vampires nearly to extinction. In order to survive vampires had to adapt to a less carnal existence, and started finding ways to hide better. Thankfully as humans made advances in medical technology methods of blood extraction became increasingly common. The Red Cross (fun fact, founded by a Sentinel) has been known to turn a blind eye to "missing" blood shipments that happen to end up with vampires.
Long lines of vampires may also have a partnership with a human family - sort of in the way noble families had ties to families of serving class folks. In exchange for a nightly meal the vampires would offer protection, housing, and compensation. This method fell out of favor in the mid 1900's when human culture shifted away from such partnerships as well. A few do remain to this day.
Most "blood bags" of the modern era are orphans, runaways, misguided youths, etc, who end up falling through the cracks of society and get picked up by a gang of vampires in need of a feed. For most vampires a 1/2 pint of blood a day is more than enough to sustain them. Most people have about 10 pints of blood in them, so if a vampire is careful they can live off of one person's "donations" of blood. However, many vampires prefer to wait and binge, feeding once or twice a week in larger amounts.
If a vampire is without their food source - be it blood, emotions, raw meat, etc - they begin to change. Most vampire lineages begin to develop Nosferatu like characteristics. Some vampires wither and shrivel. Regardless, consuming their food prevents these effects - though it can not generally reverse them.
Many vampires have the ability to "hibernate," where they can slow their metabolism to near stopping and appear to sleep for extended periods of time - even decades or centuries.
Magical abilities, like transforming into a bat, mist, hypnotism, etc are generally inherited or taught to a vampire by their sire. Using such abilities tends to "burn calories" leaving the vampire hungry, and sometimes hangry.
To become a vampire one must typically be emptied of the food source and then refilled with a vampiric food source. In the base of blood, someone must be exsanguinated and then feed off vampire blood. An emotional vampire undergoes a similar process where their own emotions are removed in their death, then they are filled with partially digested emotions from several vampires.