The VC isn't designed to lock. There are a series of fine plates immersed in a viscous silicon fluid.
VCs are rated dynamically, for example; 50NM/SEC. That is; You will need to apply 50NM to gain a slippage rate of 1 revolution per second, or 60 rpm.
50NM/sec is a typical first upgrade to an MT75 centre VC.
For a torque to occur, you need an equal and opposing load.
By removing the front prop, the load is removed from the centre differential sun gear. The planets, faced with a load at the annulus, versus an easy life at the sun, will choose to simply spin the latter, revolving inside the stationary annulus. Yes, there is the VC providing some encouragement to drive the two at the same rate, but unless the VC is rated to pretty much match the available torque at the planets against the load of the vehicle, you will simply "fry" the VC.
Remember, if you have, say, an engine capable of supplying 300NM, then in 1st gear, with a reduction of 3.3:1 (from memory on an MT75), you will have an available torque of 990NM