Grif said it well. :D Depends on context - here it is pretty equalized.
I think it ends up being of lesser concern when A.M.R.O. is throwing you at rogue wizards every five minutes.
I was always under the impression that vocum would be specifically speach. As you mentioned, the written word seems like it'd be closer to sigillum.
I imagine sigillum has some limits, although considering the specifics of vocum/motem are similarly vague I don't suppose it matters a great deal. I don't think it's ever been mentioned what kinds of movements were necessary to produce specific alchemy with motem, for example. Mind you, this did give me an idea for a character; an alchemist who manipulates ink/colour, and uses motem alchemy to create larger, more powerful symbols out of ink/colour to work for sigillum alchemy. :D
Maybe. Or a certain someone could make them a partner...? ;D
<Snipped quote by Grif of Hearts>
Deal.
I just have to think of a suitably awesome power to compliment it. ;D
Xodus, why don't you love meeeee! I need a response to that PM I sent awhile back before I can get my post up. Hmm, I might just get up the first part of the post for now so that I can at least get my intro post up.