Oh I see, sorry I forgot you wanted 26mhz and not 40.
The problem is because the esp definition does not provide any flags for 26mhz therefore the last used flags for that menu setting are used. We will adjust that in future visual micro versions.
A short term workaround might be to create a local project board.txt and add the following line. A board.txt is just a text file, the visual micro "Add/Add Code" menus create one for you or you can add to the project one yourself :-
build.extra_flags=-DESP8266
An alternative solution is to add a definition line to the boards.txt in the platform folder
This is what you currently have in boards
menu.CrystalFreq.26=26 MHz
menu.CrystalFreq.40=40 MHz
menu.CrystalFreq.40.build.extra_flags=-DF_CRYSTAL=40000000 -DESP8266
This is how it will look with a definition for the 26mhx crystal which is the default so the speed doesn't need to be set
menu.CrystalFreq.26=26 MHz
menu.CrystalFreq.26.build.extra_flags=-DESP8266
menu.CrystalFreq.40=40 MHz
menu.CrystalFreq.40.build.extra_flags=-DF_CRYSTAL=40000000 -DESP8266
The boards.txt can be found here...
C:\Users\[YOUR NAME]\AppData\Local\arduino15\packages\esp8266\hardware\esp8266\2.4.1