VLC 2.1.0 won't scale its interface to videos with a pixel width smaller than 851 pixels - at least not when the time slider is being used in "Line 1" or "Line 2" of the interface.
For years of VLC usage I've found that using the time slider together with all the other interface buttons in "Line 1" is the best option to make the player window tidy and saving screen space, so needless to say this scaling limitation becomes quite impractical when playing smaller resolution videos, which end up with an ugly horizontal window surrounded by lateral black bars and taking unnecessary screen space:
My sincere gratitude if this can be fixed in a future version. Until then, I've reverted to VLC 2.0.8 (which has a similar limitation but at least it's set at 465 pixels width, making it not noticeable for most videos).