Your "good" one is a 25p project, and your "bad" one is a 30p project
Since your source footage is 50i, and you did a 50% slow down, there is no frame interpolation in the 25p project because it is evenly divisible - i.e it gets bob deinterlaced to 50p and put on a 25p timeline which is exactly divisible by half.
With a 30p timeline project, so you have to either a) blend or b) duplicate frames (you used blend in your "bad" sample) to make it work at 50% because it's not nicely divisible because you need a 50p => 60p conversion to be 1/2 speed for a 30p project. Vegas doesn't offer higher quality c) frame interpolation (ie. generate new frames), such as after effects pixel motion
Suggestion: use a 25p timeline project, or use frame interpolation software (eg after effects, avisynth) to generate a 60p asset to re-import into vegas if you need a 30p project