Your option A is correct
You do not retroactively take the damage that you lost the first temporary hit points from, since that damage has already been resolved. You can cast false life (via the Fiendish Vigor invocation) again to gain more temporary hit points, which does not in any way interact with damage taken previously.
The rules for temporary hit points state:
Temporary hit points aren't actual hit points; they are a buffer against damage, a pool of hit points that protect you from injury.
They effectively take the damage for you, so it's as though you were not damaged.
When you have temporary hit points and take damage, the temporary hit points are lost first, and any leftover damage carries over to your normal hit points.
The temporary hit points are lost first, but nothing suggests that this has any later effect on your real hit points.
If you have temporary hit points and receive more of them, you decide whether to keep the ones you have or to gain the new ones. For example, if a spell grants you 12 temporary hit points when you already have 10, you can have 12 or 10, not 22.
This just confirms that you would have to choose between your previous 8 (if you were not damaged) and your new 5. However, since your question states that you took 8 damage, you're effectively choosing between 0 and 5, or rather, you have no temporary hit points, so you just get 5.
This also proves that it's not option B because you have no temporary hit points anymore, so you cannot choose between your previous 8 and your new 5, because you currently have 0 (i.e. you do not have any temporary hit points and therefore have nothing to choose between; you just get 5).