The humanoid is protected
The bite creates undead the following night after ground burial
The bite of a vampire (MM, 352) states (my emphasis):
A humanoid slain in this way and then buried in the ground rises the following night as a vampire spawn under the vampire's control.
A vampire spawn (MM, 354) is classified as:
Medium undead, neutral evil
So, should the humanoid be buried after a bite, they would rise the following night as an undead.
Gentle repose prevents that
This is where gentle repose (PHB, 245) comes in:
For the duration[10 days], the target is protected from decay and can't become undead.
In this case, the effects of the vampire bite are RAW completely prevented. Since the Vampire bite's text specifically states that they will become a spawn on the following night, if that night they are protected, then they have passed the gate and are no longer at risk. The fact that the spell continues longer doesn't really change anything as the first night gate has passed and the corpse will not rise as undead after that.