Answer:
It is recommended to combine the differential output with a balun, which can significantly reduce second-order harmonics. Figure 39 in the datasheet presents a suggested circuit, though a discrete SMD balun can also be used for this purpose.
Increasing the Vvco supply voltage might provide additional improvement; however, this approach has not been thoroughly tested, so the actual effect remains uncertain. While Vvco is typically recommended to match Vdd, it can be increased up to 3.6 VDC if needed.