I had to mod the throttle sleeve by cutting away about 1/32" of the area between the grip end and the throttle cable end holder of the sleeve. You have to make the room so the vista cruise fits between the switch housing and the throttle sleeve.

You use a Dremel with a small coarse grit drum sander. You have to off set the new oversize hole just a little towards the hinge rivet. Make sure you use something to hold the Vista Cruise open about 1/8" while you re-cut the 7/8" hole. Recut so the vista cruse fits over the throttle sleeve slot, this will let the Vista Cruise clamp on the throttle sleeve. Then take just a little metal off the hing rivet head and the crimp on the back of the rivet.

You want the ISO throttle sleeve to move free when in place, so you have to work slow and check your fit till ya get it just right. You will need to remove the right angle from the area of the Vista cruise that would be used to keep it from moving. This will leave the place for the stop to hold the vista cruise in place when installed. Check out the Picture. Note how the tab is mounted to the throttle cable restraint.

Make sure the throttle moves Free and then adjust the set nut to get the Vista Cruise to lock the throttle. But you should be able to twist it back with a bit of force in case you need to turn the throttle speed down in a hurry.

Just work slow and be careful the ISO grips ain't cheap. so don't screw up.

I also added a Picture of the Vista Cruise on a stock Drifter Throttle all the same applies.