I’ve been witness to a fair few interesting events brought on by the use of V/S at altitude. This is not to say it can’t be done but why increase your workload and the possibility of something undesirable happening?
Approaching optimum flight levels and above, buffet margins are considerably reduced and you need speed stability much more than anything else. Excess thrust is also reducing and in many latitudes you are often close to the tropopause, where there can be marked fluctuations in wind and temperature over a short vertical distance.
Modern aircraft, especially FBW ones, are very good at masking the reality that your flight envelope has shrunk considerably compared with lower down and the ability to climb/manoeuvre is similarly restrained. What they can’t do is override the laws of physics and if you get too slow, you’re going down again, whether you like it or not. LoC.
If you want to get to your cruising altitude in the shortest time, keep climb thrust on and fly the aeroplane-recommended optimum speed/mach until the level is reached. Using V/S, if the speed goes above optimum, thrust reduces and if the speed gets low, drag increases. Both effects will increase the time and distance required to get to cruising level.