I use google keyword tool or the google trends. check the tag after I receive the following: For Google the preposition is nonsense:
example: travel to Vietnam, travel in Vietnam, Vietnam and travel, Vietnam travel on Google understands that all 1: Vietnam travel.
If we examine the human perspective this is google the missing.
I check the number of search by keyword: travel to Vietnam, travel in Vietnam, Vietnam and travel, travel on Vietnam has one result: local monthly search: 110,000 / month.
Need share everybody