The Braton Vandal can be obtained from two different ways. The first option is to trade with other Tenno to obtain its different parts. Expect high prices, because is a weapon really complicated to obtain. Even though, as always, this will depend on your trading skills.
The second way is to go to the Sanctuary and do the mission Sanctuary Onslaught Elite. Here you face very powerful enemies, but the rewards are worthy. If you choose this way do not despair and keep fighting, it is a matter of time to obtain what you want.