rMBP 2015 - external Dell monitor issue

I just hooked up my refurbished 2015 rMBP (running El Capitan) to a Dell U2410 external monitor using the VGA cable and VGA-to-Thunderbolt adapter. The problem: the Dell display does not go to full resolution (1920x1200), it displays at 1280x1024 max res. This same ext. display/cable/adapter ran at full res when connected to: my 2012 MBA (Mavericks), my GF's 2011 MBA (Mavericks) and my GF's 2014 Mac Mini (Yosemite). I spent a lot of time on phone w/ Apple tech support to no avail. So is this a problem with:

1. El Capitan

2. the refurbished Macbook

3. the Thunderbolt 2 connector (does it require a special adapter - different from Thunderbolt 1)

The display/cable/adapter are NOT the problem, they work on other systems. Will test MBP tomorrow at work on an Apple 27in Thunderbolt display to see if there are any issues there. Any help greatly appreciated.