How does the Cloak of Resistance’s caster level requirement function if a crafter does not meet its prerequisites

If you are not bypassing the spell prereq for the item, you are forced to 'cast' it.

http://paizo.com/prd/magicItems/magicItemCreation.html

From the creating wonderous item section (similar text can be found in almost all other feat specific sections).

If spells are involved in the prerequisites for making the item, the creator must have prepared the spells to be cast (or must know the spells, in the case of a sorcerer or bard) but need not provide any material components or focuses the spells require. The act of working on the item triggers the prepared spells, making them unavailable for casting during each day of the item's creation. (That is, those spell slots are expended from the caster's currently prepared spells, just as if they had been cast.)

so in your case, each day you spent crafting the belt, you would need to use a copy of the scroll or have someone around who can cast it for you. Otherwise, you have to add +5 to the DC of the check.

Only [...] the various prerequisites required of the new ability to be added to the magic item restrict the type of additional powers one can place.

You are not creating a vorpal sword; the vorpal property is already on the sword. The relevant rule, quoted above, indicates that only the prerequisites of the new ability are relevant in this case. Thus you do not need to meet the prerequisites of the existing properties; you aren’t creating those, and the closest thing to saying you need them says:

Note that all items have prerequisites in their descriptions. These prerequisites must be met for the item to be created.

You aren’t creating a vorpal sword, so you don’t have to meet the prerequisites of vorpal. You are creating a new flaming property that happens to be placed on an already-vorpal sword.