The three major concerns might be seed age, potting mix issues and planting depth. Temperature should be kept on the cool side but I've had good germination at 80*(high end) with my onions.

If onion seeds aren't stored frozen you might only expect 3-6 months shelflife. They are one of first seeds to loose ability to germinate with age.

Why don't you use a reputable potting media to eliminate a host of potential problems. Your mix is likely to introduce fungal diseases that attack seedlings and it would appear to be on the acidic side based on ingredients.

Since seeds are relatively small don't cover them with too much media (1/16- 1/8" is plenty). I press the dry media over my planted cells prior to soaking to get good seed-media contact. Air pockets allow seed to dry during germination. I would avoid pure vermiculite for that reason.

Once you have the seeds planted and watered set the pots/trays into a kitchen garbage bag until they germinate. That keeps the media evenly moist throughout the process. Even after they are growing don't allow media to dry totally. Onions are not tolerant of excessive drying.

Thanks for the information. The seeds are a coupleof years old, but they've been stored in an airtight jar in the refrigerator.

Other problems aside, I think I planted them too deep and didn't pack the soil. I'm having good results with other seeds in this soil mix.

I take it that onion seeds are trickier than other types. Is this why so many people use sets?