This trade depends on a couple factors (don't you hate when you don't get a straight yes or no answer?
The main question here is: who are you playing at ss at the moment? If you have someone serviceable, I'd probably hold out for a slightly better offer - if not, then make this trade as soon as you can.
I agree with your analysis - you may actually be being a bit conservative. I'd rank Tejada somewhere from 4-6 among shortstops: behind A-Rod, Jeter and Nomar, and on a par with Rollins and Uribe, depending on your needs (some may disagree with these rankings, but that's part of the fun). I also think you're selling Castillo slightly short. He's an rbi killer, and his average isn't where it should be, but he's an absolute steals machine. I'd look for around 50 sb minimum this year - his current pace is somewhere in the neighborhood of 70. In rotisserie (yup - got it right
), that's huge. Can you spare 40 more sb's?
In sum, you're giving up two very valuable players (my rear is firmly on the Beckett bandwagon, although I'm not among those zealots who expect a Cy this year...) for one terrific guy and a good but not great 2b. If you'll make a net gain in the standings this way by replacing a weak ss, you'll come out ahead. If not, you might try offering Sheets instead of Beckett and see where that takes you (again, assuming Beckett's blister problem doesn't hamper him the rest of the way).
Try inserting Tejada and Vina's stats into your standings instead of your current middle infield and see whather you'd wind up with more or less points. This won't be exact, naturally, but might give you a usefull ballpark estimate. And let us know who you have at short - this trade looks pretty tough to gauge.