Within SkyTeam, awards on DL are available for partner miles only if DL has basic low availability, which is relatively rare. DL elites and even DL AmEx credit card holders get various levels of enhanced availability for low domestic coach award space.

The advantage of crediting on AS is that you can credit and redeem many different carriers through them, including AA. It's a nice niche program that's very useful for people. IMO this overrides your desire to transfer your credit card points into a FF program. I would think you want a second USA FF program, ideally UA, that will accept your credit card points. That will give you a lot more flexibility for redemptions IMO than attempting to use either KE for DL flights or VA for Virgin America flights (which is a very restricted route network).

Note that I don't know what the arrangements are for VA miles to be used for redemptions on DL flights, but I assume that regular low availability would also be required.