To begin with, how long is it since anyone says this place was open? The addressing format currently listed on happycow is no longer in use. There is a reference to Mango Tree Cafe here
This has the new address format, namely E4-310 Foch. Indeed this is 1 block away from my hotel in Mariscal. The addressing appears correct as it still also has the old 721 on the front of the building. However there is no indication that this is the Mango Tree Cafe. I've asked at hotel reception and also the tourist police and no one has heard of Mango Tree Cafe so I suspect it is closed. Not the end of the world for vegos round here if it is as there is at least 3 and probably a 4th all vego eatery in the neighbourhood.