In the past Turn 10 has released themed vehicles to support other game releases, but the last time this happened was when Halo 4 was released and a Warthog was added to Forza Motorsport. Recently, Turn 10 released a new update for Forza Motorsport 6 and it added some new vehicles to the game.
Like previously mentioned, the new vehicles being added to Forza Motorsport 6 are hemed. The new vehicles have a Fallout 4 theme - that's not to say that these vehicles are actually from Fallout 4, but their designs are very Fallout-esque.
So far only one vehicle has been added to Forza Motorsport 6 - this vehicle is the 1956 Ford F100 and it has a Fallout style paint job. The second vehicle has not yet been released and screenshots of this have not been shown yet either. Fortunately, Turn 10 did mention that the second vehicle will be the Chryslus Rocket 69 coupe and it should be available in Forza Motorsport 6 "soon."
Whilst you can't drive any cars in Fallout 4, you will be able to drive a couple of them in Forza Motorsport 6. The two new vehicles that are being added to Forza 6 will be available for free - all players will have access to them and you will not need to purchase them as DLC packs.
So far it looks like these two vehicles will be the only two Fallout vehicles available in Forza Motorsport 6, but it's still cool to see just two new themed cars getting added to the game.