Yes, you can install Vainglory onto an SD card if your device doesn't have enough space to fit it. Vainglory requires a little over 3.2GB total, and the location you install to should have at least 2GB left over after installation so the device has space for flex memory.
If you can install directly to your device, please do so because Vainglory is very performance hungry and adding an extra layer of reading/writing doesn't always make the game happy.
If you do use an SD card, we highly suggest making sure it has a very fast read/write speed.
Ideally, it should be capable of handling 4K video capturing.