At matchpoints, my left hand opponent opened one diamond. Partner doubled (takeout). Right hand opponent stuck in a one heart bid, which theoretically "took me off the hook."
I elected to bid one spade with ♠QTxxx ♥JT ♦Jx ♣Txxx. This was now a "free" bid, which usually entails eight points (partner had expected 10). Factors in my decision to bid light included the fifth spade, three tens, and two doubletons in the opponents' suits, which would make "transportation" easy, despite my low point count.
Partner raised me directly to four spades with an 18 count consisting of ♠AKJ ♥AKxx ♦xxx ♣QJx, leading to a result of down two. In partner's shoes, I would raised to only three spades , holding only three spades. According to these tables of data an eight card fit with a 18-4 high card point distribution is a decided favorite to make nine tricks, while a nine card fit is slightly favored to make 10 tricks.
Given that we were vulnerable, our down two led to a "shared" bottom (with others). I would think twice about doing this again when vulnerable. But if we were not not vulnerable, or partner had bid only three spades, -100 would have gotten us a shared top against opposing part scores.
So was vulnerability the deciding factor in how I should have responded, or were other considerations?