So, hitting vehicles in combat. I have to say, I didn't particularly like the 5th ed way of doing it - straight To Hit values were a very hamfisted way of dealing with the problem, as it didn't take into account the skill of the attacker at all. Vehicles that went at Cruising Speed were also really, really hard to hit!
However, I don't like 6th ed's Ws0 Or Ws1 system either. For a start, a raider that has gone 30" in a turn should not be hit on a 3+ by a grot - nor should it be as easy to hit as a monolith that moved 0.5". So, new system:
Hitting Vehicles in combat:
If a Vehicle is immobilised or didn't move at all last turn, it has Ws0 (and is therefore hit automatically in close combat).
If a Vehicle moved in the previous turn, it is considered to have a Ws value equal to the total distance it moved, divided by 3, rounding down. For example, a rhino that moved 10" last turn would have a Ws value of 4.
This distance must be the distance it moved from the starting position to its end position in any one phase, measured from the central point. Moving around in circles or back and forth does not add to its Ws. For example, a rhino drives 12" in a phase, but actually only goes 10" from its starting position to its end position (as it drives around a wreck), thus it only has a Ws of 3. It that rhino then flatouts in the shooting phase this may increase its Ws further.
Note that starting and end position and total distance is calculated separately for each phase - in the above example, if the rhino went 6" sideways, its total distance from the original start point may not be very far, but its total Ws would be 5 (10/3 + 6/3). If the same rhino had gone only 5" effective distance in the shooting phase, it would only have a Ws of 4 = 3 from the movement phase and 1 from the shooting phase.
No Vehicle may have greater than Ws10 for any reason. If a Vehicle goes further than 30" in any one turn (calculated as above), then all successful hits against it in close combat must be rerolled. This normally only applies to Flyers, as going at these speeds is otherwise impossible for conventional vehicles, but some exotic wargear does allow for very fast movement.
Basically, I think this would much more accurately represent how speed would make the vehicles harder to hit. You have to go at a fair clip to avoid having trained soldiers (i.e. marines) hit you easily, but you have to go like a bat out of hell to make it very difficult for them to hit you. The only difficulty is that there would be a fair amount of book keeping on how fast everything has moved.