I was wrong. Almost all D models above the 60D support VXLAN except the 200D. I am just lucky I guess.
From the 5.6.8 release notes:
The following models support VXLAN.
FortiGate FG-30E, FG-30E-MI, FG-30E-MN, FG-50E, FG-51E, FG-52E, FG-60E, FG-60E-DLS,
FG-60E-MC, FG-60E-MI, FG-60E-POE, FG-60EV, FG-61E, FG-80D, FG-80E, FG-80E-POE,
FG-81E, FG-81E-POE, FG-90E, FG-91E, FG-92D, FG-100D, FG-100E, FG-100EF, FG-
101E, FG-140D, FG-140D-POE, FG-140E, FG-140E-POE, FG-200E, FG-201E, FG-300D,
FG-300E, FG-301E, FG-400D, FG-500D, FG-500E, FG-501E, FG-600D, FG-800D, FG-
900D, FG-1000D, FG-1200D, FG-1500D, FG-1500DT, FG-2000E, FG-2500E, FG-3000D,
FG-3100D, FG-3200D, FG-3700D, FG-3800D, FG-3810D, FG-3815D, FG-3960E,
FG-3980E, FG-5001D, FG-5001E, FG-5001E1