The vault issue we had was zero cost to flood the network and kill it. The consensus was actually retained while not under attack like that. When the network did collapse (we took it down) then it did lose consensus, but it would in those circumstances. With no data (or very little) though we imagine that even those attacks would not be fast enough to kill a routing only network (alpha 3) as a bit problem is when there are nodes with lots of data, then copying the data around while the network gets much smaller fast means the vaults lose time to sync and data gets lost.
So PARSEC means we go faster and more securely, node age makes attacks harder, safecoin should make them too expensive, sort of in that order.
tl;dr Not so much a consensus issue, but PARSEC helps as it will be much faster, node age, crust fixed means alpha 3 should be fine. Then vaults with test safecoin should secure the whole thing (alpha 4).