If the symptoms change - particularly if they become less severe - as the set warms up, a dried up electrolytic capacitor is most likely. If they get worse, it could be a bad semiconductor. Freeze spray or a heat gun may be useful in identifying the defective component.
It is often easiest to substitute a good capacitor for each electrolytic in the vertical output circuit. Look for bad connections (particularly to the deflection yoke), then consider replacing the vertical output IC or transistor(s).
A defective deflection yoke is also possible or in rare cases, a bad yoke damping resistor (e.g., 500 ohms, may be mounted on the yoke assembly itself).
Where the entire top half or botton half of the picture is squashed into into the center (i.g., only half the picture shows), a missing power supply voltage, defective vertical output IC, or a component associated with it is likely bad. A bad connection or blown fusable resistor may be the cause of a missing power supply voltage.
The following are NOT possible: CRT, flyback (except possibly where it's the source for a missing voltage but this is more likely just a bad solder connection at a flyback pin), tuner (except for the famous RCA/GE/Proscan or Sony models where the controller is at fault - see the sections on these specific brands). I am just trying to think of really expensive parts that cannot possibly be at fault :-).
Note that some movies or laser karaoke discs are recorded in 'letterbox' format which at first glance looks like a squashed vertical problem. However, the picture aspect ratio will be correct and turning up the brightness will reveal a perfectly normal raster above and below the picture.