Can I take Vitamin D3 with Fish oil instead of food?

You can take your vitamin D3 with MCT alongside a fish oil supplement... both are fat-based which can support absorption.

That said... MCT & the oil in the fish oil supplement may not be enough... and to cover all basis, we would always prefer to take both vitamin D3 & Fish oil with a meal that includes healthy fat (like eggs, avocado, or nuts) to maximise absorption.

But I understand it's not always convenient, so do the best you can.