Don't know if your talking about a remote condenser, but assuming so, I would say the 3/8 line would offer less chance for pressure drop between the receiver and expansion valve than a 1/4 inch line. I'm sure the 1/4 inch line between the condenser and the receiver is too short for there to be any significant pressure drop.
The system would most likely work fine with a 1/4" liq. line as well, 3/8" is probably used to be safe. The liquid line can safety go up to 300 fpm while the condensate line is limited to 120-150 fpm to prevent vapor lock.
I had the distinct privilege to venture out this AM (Sunday) to observe one of these R22 condensing units that patch was referring to. It was a Copeland F3AD-0075 (or something like that) and the outlet of the condenser was 3/8", it reduced to 1/4" , turned up and ran 1/4" vertically about 6" into the receiver inlet. The receiver outlet was 3/8"FL. (I was making the big bucks watching ice melt so I had extra time for such observations)
The "vapor lock" term is a new one on me too, but I suspect it's referencing the plumbing term for an air bound drain line......one that's not vented properly.
The flow velocity of 150 ft/min is used in refrigeration condenser drain lines to allow for "sewer line flow" (I'm not making this up) where there is sufficient cross-section of the piping to allow liquid to flow in one direction and the vapor to vent back to the condenser in the opposite direction. You don't see much of this except on fairly large systems.
I don't have my charts handy but that 3/4 HP R22 unit at +25 SST has a capacity of about 1/2 ton. I would be willing to bet the velocity in that condenser drain line is well above 150 FT/min.