Bf 5 week old baby now vomits with formula

[An89]

My baby was exclusively formula fed since birth. At 5 weeks i have started bf...I still use formula for at least 4 feeds as baby is never full of breastmilk and can stay on me for 4hrs + but still want more.
Baby has vomited twice (not spitback) after formula. This is strange no when when he's had it for 5 weeks?

Possibly drinking the formula really fast as bottle is much easier than BF. If it’s that then a smaller teat size should help

Try pace feeding to slow it down a bit.