Understand that Vampirism was basically an analogy to Disease, and suddenly a lot of their handicaps make sense.
Sunlight
Granted vampires are associated with the dark, but being out in the sun has been associated with health for millennia.
Running Water
Stagnant (still) water could often have various diseases and parasites, but swiftly running water generally had fewer of either and thus was generally considered healthier.
Garlic
It was belived in many regions of europe during the middle ages that diseases were associated with scents. Breathing in the wrong scent (such as the scent of decay around a corpse, the scent of refuse, the scent found around those long ill, etc) could cause illness. A strong scent was believed to effectively replace any weaker scents present (as the weaker scents could no longer be smelled). Thus a strong smelling herb / plant was generally seen as healthy (so long as it was not poisonous or had a bad reputation).
Salt
This was a great preservative, well known for keeping meat from going bad - and thus seen as 'warding off' disease. Initially - and still in some places - salt was seen as an effective barrier against fiendish creatures such as fae and undead. A line of salt across an entrance (or a circle of salt around something) was seen as able to prevent 'evil' from crossing / entering. This probably lead to the idea that if a vampire comes upon a pile of grains / seeds it must stop and could not pass until it had counted every one.
Holy Water
This is obvious; the holy repells the damned. It could be a holy sign, a cup of holy water, a holy wafer, a blest picture / icon / statue, etc.
Even Gaseous Form could be attributed to this, due to the fact that swamp mists, mists in grave yards, etc were generally seen as potentially diseased. And note that when the vampire calls upon the creature of the wilderness they call upon wolves and rats (bats came later). The former were seen as fiendish, and the latter were strongly associated with disease.
And, of course, those that were bitten were infected with vampirism - becoming themselves contageous with it after it had time to incubate (ie: after their rose into unlife).
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So simple rain water would not be enough, and gutter water / barrel water would likely be quite the opposite, but a stream or river would be ideal (as would the ocean, incidentally).