During my last flight, I didn’t even keep the nib upright, and still didn’t have problems. I had multiple vacuum fill pens with me, including the VAC Mini and Pilot Custom 823, and none of them leaked.
Congrats! I’ve just finished journal writing with my 700VAC R and Iroshizuki ink! It’s my favorite travel pen, both because of the large ink capacity and significantly decreased chance of leakage during flights.