Well, you already own a very decent zoom lens for everyday use. As per landscapes, the 18-105VR is a solid performer and quite sharp at wide angle, besides, the 3.5 aperture vs. 2.8 of the Tamron it's not such a great difference.
If you shoot portraits, then the 50mm f/1.8 seems the obvious choice for me, it's more than a stop faster than the Tamron and way sharper at large appertures (the Tamron struggles a bit at 50mm when wide open).
Count me as one totally in love with primes, I've got a 18-200VR for everyday use and I love it, but no zoom can match the tack sharp pictures I can take with my 35mm F1.8 (and don't even get me started on how fast primes perform indoors). The 50mm F1.8 is in the top of my list as of today.
That said, if you're interested in a fast general purpose lens, then the Tamron it's a great choice, as shown by its many positive reviews.
P.S. BTW, the Tamron is a 17-50mm, not 18-55mm