The Vespa GTS 250 design uses a steering column with a one piece fork column that inserts through the body upwards. The headset (the scooter part with the handlebars) is kept tight to the top of the fork using a horizontal pinch bolt approach at the neck, tightening the headset frame neck to the column part sticking out of the top of the frame. This pinch bolt can be loosened to make alignment adjustments as needed.
Place the vehicle in a clear workspace. Straighten the headset by hand so that the front wheel and headset are pointing straight forward. Step back and look at the vehicle to eyeball how much adjustment is needed left or right versus the position of the front wheel.
Take a socket wrench and set it to a loosening direction (spinning left is loose, right is tight). Apply the wrench to the neck bolt while keeping the headset relatively still or secured by hand. Loosen the bolt enough to freely twist the headset free but not lose the bolt and nut keeping it on the column.
Place yourself in front of the vehicle facing the headlight. Use the front wheel as a guide and line up the headset so that it is horizontal by 90 degrees to the front wheel’s direction. Tighten the headset with the socket wrench turning the neck pinch bolt just enough that the headset doesn’t twist on its own off the fork by being completely loose.
Sit on the vehicle and look at the position of the headset relative to the front wheel position. Turn the scooter on and let the headlight beam shine against a wall. Confirm the centre of the beam is consistent with the direction line of the front wheel line going forward. Repeat the loosening, hand adjustment, and tightening until satisfied with the straightness of the headlight.
Use the socket wrench to tighten the neck bolt to full tightness again. At the first feeling of hard resistance try to turn one more quarter turn of the wrench. Do not over-tighten.
Have friend help with checking alignment by having him stand in front of the scooter while you are sitting on it and making adjustments. This gives you two sets of eyes to check alignment rather than just your single perspective.
Not aligning the vehicle properly can result in steering error which can result in an accident, particularly at high speed or in turns. Regularly checking steering alignment can avoid this problem.