Addition to TimB's answer:
Expelling the waste might also be taken care of differently - if the vampire were able to digest the blood quickly enough, it could simply drink and return. This would require a powerful blood filtration system, much more so than human's, but it might be doable. As soon as the blood is mostly stripped of its nutrients, it can be returned back to the victim.
In fact, if you also add some enzyme to force the victim to feed more nutrients to their blood, you could get much more then 900 kcal per litre - and in fact, do so repeatedly. While this would prolong the feeding process, it would also mean that he'd be able to feed comfortably on just one person, and without doing harm to the victim, other than certain level of feebleness.
So - induce a "need more energy" state in the victim, which will make it pump more nutrients into its blood, including metabolising fats, filter the nutrients out as quick as possible, and return the blood back. Ideally, in an outright loop, similarly to how human kidneys work, just on a bigger scale. You don't get water toxicity, you don't gain 5kg in weight, you get much more nutrients than from just sucking the victim dry, and you don't kill the victim - win, win, win, win.
In fact, this could even be seen as a symbiotic relationship if you add some beneficial effect of the blood filtration. Perhaps the vampire could also improve the immune system of the victim temporarily (might have evolved to protect the vampire from infections through victim's blood). Maybe it can filter out stuff that can cause harm to the victim, e.g. poision. It could help prevent various diseases related to blood content, e.g. some coronary diseases, artery clotting... At the very least, it could be used to help the host stay slim, by draining a certain portion of his nutrient supply - the ultra-cool vampire diet! In fact, I could easily imagine a society with nobles hosting vampires specifically to show off their wealth etc. They would eat 2-3x as much as "poor" humans, and dispense the extra to their vampire symbiotes, thus staying well in shape.
The symbiotic relationship would thus be beneficial to both the human host and the vampire. Feeding gets easier for the vampire, because it's suddenly a helper, rather than a mugger (think medicinal use of leeches). Humans might get a host of benefits, including improved health and easier weight management. Imagine how much people would pay for a vampire service today, just to get rid of that extra fat! :D