I have some efests that came with a purchase I made, I hated them. They really offer very little in terms of wattage or run time.
With that said, IMRbatteries are my primary vendor, I mostly use Samsung 25s but I run rather low wattage, they are fine for my needs. The "don't use in vaping applications" is a standard warning for any batteries purchased from IMRbatteries, and I can't say I blame them. It's a blanket, legal covering of butts, due to vaping explosions that have happened.
But, you can do better in a battery, and it sounds like you are headed there. I think it *may* be safe to use the efests until then, I just wouldn't expect them to be excellent performers in any way, and keep your wattages low. I got rid of mine as a) don't know how old they were and b) can I just emphasize again how much I hated them, LOL.
Anna