I did a few checks and notice that Vodafone connects using GSM, Labara LTE. Also Labara connects to a different mast! Not what I was expecting.
Im not sure what you're using to come to that conclusion, but GSM is used to denote 2G, so it's highly doubtful that Vodafone is only connecting via that (it's really only fit for voice and sms).
LTE is 4G and is the most likely connection these days in urban areas, the other is WCDMA which is 3G.
LTE runs on a number of frequencies, it's possible that Lebara doesn't have access to all the frequencies that Vodafone use.