Some, if not many vertical deflection circuits use a push-pull
oscillator setup. One side may have failed I agree to check for bad
caps. If you do replace, try to find the same µf rating with a larger
case/higher voltage, if the PCB layout allows it. Be sure you stay
within the same error tolerance. This will put a beefier component to
possibly patch a circuit weakness.

Another thing to look for is the deflection yoke, or it's control.
While the yoke rarely goes bad, they do from time to time. Top and
bottom deflection should be more or less equal, so if you can isolate
the upper coil set from the lower, be sure they are recieving proper
voltage. If not, the output can be hampered by a variety of issues,
but think simple. I have run into a similar problem which was fixed
by simply repairing a bad solder joint for the yoke's plug on the
PCB. Any discoloration of the windings is an extremely obvious
problem.

What you likely have is a bad component, not a faulty adjustment.
Adjustments go out of shape very gradually as a result of wear. An
instant failure or a rapid one indicates a component fault. Forget
your TV's internal modes because even if you did find it, you will
not find such an adjustment.

Service mode simply kills vertical deflection for aligning guns,
adjusting color drives/chroma, and miscellaneous troubleshooting. All
service mode will give you is your entire picture compressed to a
single horizontal line on the screen. This is not what you are
looking for.

If you aren't proficient with electronics, take it in to be repaired
instead of meddling with controls that are already properly set. One
does not tweak-out the base vertical size adjustment to make up for
vertical collapse.