VGA impedance matching
Since the length of a 50 Ohm connector is &lt;&lt; 10% of wavelength at say roughly 100MHz bandwidth of  2 million pixels refreshed at 60Hz Vertical sync rate, so no conversion or adapter is needed.  

50 Ohm BNC connectors I believe were  used on old NEC CRT RGB monitors without issues but used 3x 75 Ohm cable and possibly other cable for separated sync.  

... as Brian points out they must have been 75 Ohm BNC. This is due to ratio of diameter of  ground and inner pin being higher which is what determines √(L/C) ratio and thus Z.   

... Later, Dan points out that  BNC's 50/75 Ohm have the same pin size.  So I checked it out and they are indeed the same, so my memory was ok. (ha)



(this is not an endorsement for cheap err. inexpensive Pasternack connectors, that often do not have return loss specs at their rated GHz but for BNC it is fine.) GHz RF connectors are micromachined critical parts. Don't go cheap unless it doesn't matter.)

Suggestion

Get 75 Ohm connectors for better coax fit if that matters.
https://www.pasternack.com/75-ohm-bnc-connectors-category.aspx