I had FeLV hit my fully vaccinated indoor boy last summer. Vaccinations are only about 60% effective and 25% of exposed cats can contract the disease. If you do the math, that makes it a 1 in 10 chance of a vaccinated cat catching the disease. I had 11 vaccinated cats and 1 caught it - the odds were against him.
At the time this hit, we lost a total of 8 feral cats to FeLV, and 3 disappeared about the same time so we assumed it hit them also. The colony dwindled down to 4.
You face the tragic dilemna of "Individual Cat versus Cat Nation" with FeLV in a feral colony. FeLV doesn't usually pass thru casual contact. It takes a fight, a mating (which is violent), or prolonged interaction for it to transmit. Some cats are simply more prone to catch it than others. If any of the colony is unneutered, you are at risk. If any of the colony members are bonded with each other, you are at risk. If any of the colony interacts on a regular basis with your indoor/outdoor cats, you are at risk.
Last summer was absolutely tragic for me. After months of agonizing over it, I personally chose Cat Nation, but you need to do what you are comfortable doing. No one can tell you what to do here - you need to go with your heart.
Please feel free to PM or e-mail me if you need to bend an ear!!