I just checked and Vertical Sync was activated - also the NVIDIA driver shows it is set by the application.
I've just found what my mistake was: the lower "Auto find", where you can enter the IP of the phone that has the toy connected wasn't divided by dots but dashes ( - ) and had a weird "Lovense.club" in front of the port number. After I removed that and turned off the iOS Network Setting "limit Tracking of IP address" or something along those lines (I use my phone in German but with English apps, it's a little multilingual ;-) ), the Game found the devices registered with my phone
I only felt the vibrations during matches and in the Holojerk since I didn't progress yet in the Story.
I miss to feel some vibes when the sexy minions fuck in the Secret Base though and maybe with the one animated image at the HoloPics with the images where one of the main characters jerks off to his superhuman boyfriend's pics.
It's weird though: the horizontal lines only appear when I change the level in the Secret Base, btw., despite Vertical Sync active.
Haven't seen them in the Holojerk Animations yet. So maybe that narrows the thing a bit for you?
Still feels really great to play it that way...a little distracting during matches, but also a level of real life challenge and "immersion" if you will ;-P