Yep - Vertigo for sure - have had exactly the same in a Gazelle with the doors off. It's just a psychological thing, normally the doors prevent you from seeing straight down so it is very different when you have them removed.
I don't do heights and get Vertigo but I can fly a helicopter in the mountains with rock on one side and thousands of feet of fresh air on the other - hasn't stopped me from flying for 35 years - crack on.