Fortnite is doable on Ryzen Vega graphics. However, even an entry level gaming card like an RX 560 will give you noticeably better performance.
The Ryzen comes with several conditions. You need two sticks of ram, for dual channel operation. You need fast ram, at least 2933 but go for 3200 if you can. You'll need to make sure the bios of the motherboard is up to date, or update it yourself, if it's the 3xx series.
On the other hand, I have a hard time recommending anyone buy a 4560 at this point in time. I personally would not buy anything less than a quad core. If you're going Intel this means an i3 8100 at minimum. Fortnite is well optimized, so a 4560 can handle it but it leaves you no extra performance in the event you want to play something more demanding.