VORP/RARP are calculated from league averages at that position. RARP (runs above replacement player) is simpler so I'll explain it to give you an idea of how it's done.
First you look at a player's EqA* (equivalent average). This takes all of the player's offensive contribution (H, XBH, BB, HBP, SB, CS, SF, SH) and puts it into one number.
*Note, before you start questioning this number, consider this. Statisticians have looked at every season of team statistical data and determined which statistics actually lead to more runs. This is called correlation. Correlations range between 1 and -1. A correlation of 1 means that every time a stat increases, then the team's runs scored increases (-1 is a perfect increase/decrease relationship). A correlation of 0 means that the statistic has no output on runs scored. Here are the correlations of BA, OBP, SLG, OPS, and EqA.
BA - .828
OBP - .866
SLG - .890
OPS - .922
EqA - .928
Once the you have the EqA, you convert it to EqR (equivalent runs). Replacement level has been statistically determined to be about 73% of league average. So, you take the league average EqA for that position, take 73% of it, and convert that number to EqR. The difference between the player's EqR and this number is the RARP.*
*Note, I keep saying you, but you don't really have to do all these things to find these numbers. Non-subscription users can access all of this information at Baseball Prospectus.
Why did I go to the trouble to explain this? To show that part of your assertion is right. These numbers are computed from league averages, so more elite hitters at a position will raise the league average, making the replacement level (RL) higher. That means that the replacement level for SS is lower. By these metrics, if you were to take Manny + a RL SS it would be about the same as Miggy + a RL LF. However, teams aren't limited to replacement level when building their roster. Last year there were 26 COFs with a 30+ VORP, while there were only 5 SSs, meaning it would be easier to get a higher combination of the two postions with Tejada.