Something like this should work:
onready var persistFile = "user://savedata.txt"
onready var saveHandle = File.new()
func saveUserData(dict):
saveHandle.open(persistFile, File.WRITE)
saveHandle.store_line(to_json(dict))
saveHandle.close()
Then, call saveUserData()
like:
saveUserData(save_vars)
Note, the above is extracted from a working persistence script, so you may need to season it to taste.