This is my suggested 'quick and dirty' movement system:
For any given combat, either have a rough map of the battlefield or know just enough about the scenery to answer the following questions:
- What kind of terrain is it?
- Which combatants are protected by the terrain in some way?
- What factors are there that will add botch dice to one (or more) combatants?
Terrain is anything from cold stone dungeon floor to muddy swamp to rocky hill. Have a think about how easy it would be to move through. An open field is fairly easy, a muddy swampy root-entangled swamp considerably less so.
Combatants protected by terrain includes things like archers on a hill, or on the far side of some kind of lake, or even soldiers on a battlement.
Botch factors include things like rain, slippery ground, low visibility, etc. Add one botch die to all combatants (who are affected) for each item.
At the start of the combat, work out who is close enough to melee (if applicable) and who isn't. Typically speaking anyone within about 15' of each other are 'close enough'. If the group are charging entrenched archers, then no-one is. If there is a wizard on a hill and some summoned wolves sent into the fray, then the wizard is not in fisticuff range while the wolves are.
To engage in combat with someone who is in rough fisticuff range, it's simply a matter of saying 'I attack that guy!'. Automatic engagement, proceed immediately to attack rolls.
To engage in melee combat with someone who is not in fisticuff range, closing is required which involves a Quickness + Athletics - Encumbrance roll vs. a difficulty value based on the terrain and any hazards in the way. Open ground: 0+. Climbing a siege ladder: 15+.
Groups use the worst Quickness + Athletics - Encumbrance of its members, not that of the Vanguard.
If a combatant fails to make the roll, they can try again next round with a +3 bonus if they attempt the same thing. If they succeed, they have closed that round and may proceed to make attack rolls.
Voice range is ~30', but in most cases combatants involved in any fight will be within voice range if the magus isn't afraid of shouting really loudly.
Magi can quickly alter the battlefield conditions, changing the target values for those athletics rolls for all or some combatants.
I'm not a huge fan of ranged combatants being able to engage targets, so no 'you must disengage to move' for me. Tried it once, just felt clumsy. I'd instead try adding +3 to the Quick+Athletcis difficulty and +1 botch die if the closing group are being attacked by ranged combatants.
Don't underestminate the power of adding botch dice. I've had combat botch dice climb as high as 6 without any kind of supernatural aid - high enough that the players get a real sense of risk.
Also, don't underestimate the importance of concentration checks for magi while in battle. Magi can't even move and cast in the same round without requiring a concentration check.