RoS and Vanilla will be separate games, no interaction at all.
Can you imagine how much ilvl70 items with -10 reduced level requirement roll on them would be worth if everything was kept together?
Who said so?
Separating them would cause more bad than good imo. Your one issue is valid though, but they'd probably be able to solve that without migrating every vanilla player to a separate world. They could just flag items as being from RoS and therefore unable to be equipped by a vanilla char. Or even simpler, avoid that altogether by not allowing lvl70 items to roll more than -9 lvl req (and lvl69 items -8 lvl req and so on).
edit: And even if they did nothing against that, I don't see the economical issue: It's not like -req would be super-rare. It'd only ruin gameplay for vanilla chars. Like -req weapons already do for lvl50 chars
Says history with almost every expansion to almost all non-MMO games ever. I suppose D3 could be an exception, but I'd put money on having 2 separate games, AH's, etc.
Who said so?
Separating them would cause more bad than good imo. Your one issue is valid though, but they'd probably be able to solve that without migrating every vanilla player to a separate world. They could just flag items as being from RoS and therefore unable to be equipped by a vanilla char. Or even simpler, avoid that altogether by not allowing lvl70 items to roll more than -9 lvl req (and lvl69 items -8 lvl req and so on).
edit: And even if they did nothing against that, I don't see the economical issue: It's not like -req would be super-rare. It'd only ruin gameplay for vanilla chars. Like -req weapons already do for lvl50 chars
http://eu.battle.net/d3/en/profile/Sol77-2972/hero/66110450
Lvl 50 is temporary, lvl 60 is permanent (to a degree) ;).