I have used Vickerman straight line dies for 35 or so years now, currently for .22, 6.5, and .30 calibers. They have their limitations, but are a sound design and they work. Many a time I merely bought sizing dies and used the Vickermans to seat. My biggest gripe is what you are describing.
What I do, and this is kind of half-assed, is I take a cartridge case and cut it off right above the web, turn a steel plug to fit in the hollow (with an outside diameter matching the case diameter) an inch long or so and solder the two pieces together. That makes for a solid platform extension with which to push the short cartridge up into the die. The rim of the Jerry (oldotter)-rigged extension fits into a regular shell holder. For the .250 or .300 just use an '06 case head. The sliding sleeve inside the die (which governs straightness) grips the neck of the case and keeps everything aligned while you drop a bullet in from the top/side, continue pushing the case up until bullet is seated. Done.
One thing I found with Vickerman dies is they are machined to pretty tight tolerances, and bullets larger than nominal diameter (ie: .310 instead of .308) won't fit in. That makes them troublesome to use with cast lead bullets as they are most always somewhat larger than nominal. I actually made a new slide for my .30 to allow for that, but it was admittedly a PIA and the end result doesn't work any better than a good competition seater die.
Still and all, they are dies I will always have and use.