To cast a spell, you must pass a Casting Check.
For your Casting Check, you roll 1d6 for each Casting Level and 1d6 for
each Fatigue you spent (up to a limit of your Casting Level). This check
must meet or exceed the Power Level of the Spell.
You can't spend so much Fatigue adding d6's that you collapse (i.e. make your Fatigue go above your Fortitude).
If you fail your casting roll, you take a Fatigue (this may make you collapse).
Concentrating on a spell requires a Casting Check every minute at half the spell's PL, rounded up.
I've never liked Vancian magic. It doesn't capture the feel of magic I like, where spells take a physical toll on the caster. Spell point systems have always struck me as unparsimonious, and systems using hit points to cast have always seemed too harsh (along with the problems of magical healing creating spellcasting power).
Including Fatigue alongside physical damage provides a convenient mechanism for physical weakening of the caster.
The solution I like to Vancian Magic is a system where casters have a Casting Level which defines how many d6's they can roll for their "Casting Roll". Each spell has a power level that needs to be achieved on that Casting Roll. A failed roll indicates that you take a point of Fatigue, and casting the spell will take another round.
If you want to roll extra d6's on your casting roll, you can - but each one costs a point of Fatigue. Decide before you roll how much effort you want to put into the casting.
The first problem with any stamina or health based magic system is that Constitution or Strength become very important to Magic-Users, which is considered "out-of-type".
The first option that comes to mind is giving an extra Casting Level for high Intelligence. This immediately makes Int far, far more valuable than Str or Con for every-day casting.
The second option is a little more radical: allow characters with low Fortitude to add an extra Caster Level. I'm not convinced this is a good option, but it's an interesting thought - I'll expand on this idea further.
The third option is to have a more D&D-like cap on caster's ability to absorb damage. For example, if you start as a Caster your Fortitude is capped at 10. I don't think I care for this option, as it short-circuits random character generation a bit.
The fourth option is to make people choose class before rolling 3d6 in order for abilities. This has the potential to create some fun out-of-type play, I think.
The second problem is magical healing. I plan to solve that by having little or no magical healing available (limited to first-aid type stuff, to help people who've failed a Fortitude check, for example).
If you'd like to convert D&D spells to this system, I would suggest Power Level = Spell Level x 6, more for direct-damage spells like Fireball. This would up the potential nova power of spellcaster a bit, but at make them trade stamina for that power.
A first level caster with Fortitude 10 and Casting Level 1, would, on average, be able to cast 1.6 converted D&D spells per day before exhaustion.