The supreme court declared the Virginia poll tax law unconstitutional. By making it more difficult to vote, the state was in clear violation of the 14th amendment's equal protection clause. Voting is a right to all American citizens and shouldn't be denied to someone based on socioeconomic status.
Therefore, state laws must conform to the constitutions of Virginia and the United States. However, the United States constitution comes first above all and no matter what part of the united States you're from, all stretches of this vats land must conform to that supreme law.
This is because as a result of Marbury vs. Madison, and all other court cases and decisions to follow, the supreme court declared that the constitution of the United States is the supreme law of the land and nothing can overstep or abuse its' constitutional laws.