There are three effects of the vampire’s bite:
(a) the target takes 7 (1d6 +4) piercing damage plus 10 (3d6) necrotic damage,
(b) the target’s hit point maximum is reduced by an amount equal to the necrotic damage taken, and
(c) the vampire regains a number of hit points equal to the amount of the reduction.
All three of these effects happen immediately on a hit. I think everyone agrees on this.
The disagreement in this thread hinges on different understandings of the following two sentences:
1. The reduction lasts until the target finishes a Long Rest.
2. The target dies if this effect reduces its hit point maximum to 0.
Let’s take them one at a time.
I think the the meaning of the first sentence is clear. The reduction referred to is the reduction to the target’s hit point maximum that was caused by effect (b), and the purpose of the sentence is to say that the reduction is not permanent. Upon the target finishing a long rest, the target’s hit point maximum will automatically be restored to what it was before it was reduced. I don’t think this is meant to imply that the target’s hit point maximum can’t be restored by other methods, such as casting
greater restoration on the target, for example.
The second sentence is the one which I think is causing the most trouble, but I think it’s clear that “this effect” refers to effect (b). Given that (b), like the other effects of the bite, happens immediately on a hit, the condition for the target dying can only be met at that time. I don’t see anything to imply that, once dead, the target can’t be brought back to life by any suitable means, such as casting
revivify