I'm not exactly sure, and a bit lazy to test it, but I noticed that the game checks for that 75% health state after the damage of the grenade is applied, meaning you can snare a vehicle that is seemingly at above 75-80% health.
For example, you can always snare a full health P4 with double at nades (given they penetrate):
1st nade gets it to about 85% health (no snare)
2nd nade gets it to about 70% health, and snares it.
That's how it is supposed to work. If a vehicle goes below 75% HP at the end of the hp check, it is snared.