I am not sure about what you are saying. Are you saying spin-offs are fallout games but Tactics or are you saying Tactics is a fallout game but other spin-off are not. Also are you saying New Vegas is not canon because its a spin-off? I am confused.
All fallout games but one are canon fallout games. FO1, FO2, Tactics, FO3 and New Vegas are all Canon.
Sad news FO3 is also a spin-off for it does not continue the story of FO2. Just because it has 3 next to it does not make it a sequel.
This,
Fallout 3 follow another plot, the game itself doesn follow the events of Fallout 2,
NV on the other hand, have various things about the FO1 and FO2, plots, yet is considered a spin off
EG: GTA4, yes, it have the number 4, but it doesnt follow GTA3, the game is a continuity reboot