Sadly its not abuse of the system. Any player can use vacation mode, even as a war tactic, provided the pre-requisites are met.
It might be frustrating for a player to use vacation mode to stop you taking an oasis but its not breaking the rules or abusing the system, its using it in a way that was considered at the design stage (thats why there is a long list of pre-requisits)
Think someone might use vacation mode tactically to block you or ruin your plans, then start sending attacks, as long as they have incoming troops to their villages they cant use vacation mode. Depending on distance it should not be too complex or costly for you to always have 1 troop on its way to them at all times........