Instead of worrying about position players, the Nats should focus their energies on pitching, at the major league level, and in the minors. What they have now is atrocious. The Nats have not even one pitcher projected as a potential number one starter at the major or minor league level right now. That in itself is a very sad indicator of what kind of trouble the entire system is in.
At one time, in MLB, if a team was serious and had a "Five Year Plan" unless they had incompetent people working for them, odds are they were successful. Today with the better Free Agents signing 3 to 5 year contracts, no kind of plan is a guarantee. Building your franchise through the farm system is all well and good, but there too, you need very qualified people doing the work every day, plus you must commit some major dollars to signing the elite amateur players, not just for one or two years, but every year.
With very few exceptions, drafting high school players is a big risk. The odds of those players being successful does not increase because they have more time to develop. In a vast majority of cases, its more a hindrance. Those three extra years means the chances of them advancing fast enough to outrace newly drafted high school kids or college players, in a large percentage of case, isn't very good. Each year a team will add 20+ players to its minor league system, meaning that each year, 20 have been pushed aside, left as minor league free agents, or dealt to another team.
Regarding Kory Casto, if the Nats have any intention at all of trying to sign Soriano, then they should move Casto to second base, to give him as much time as possible to be ready when Vidro is traded, or simply not resigned. Thing is though, do the Nats have any serious plans for Casto, after all he's 24 and still in AA. If the Nats drag their feet, he'll be 26 or 27 by the time he hits the majors, and excuse my being blunt, 26 and 27 year old rookies is a sign of a very bad farm system. What would be best for Casto, is to be traded for a top level AA and younger pitching prospect. He's from Portland, OR, so trade to the Mariners, who are looking for a left fielder.