I'm currently using a 18-70 and a 55-200VR setup. It is a cheap and effective combination. The 55mm provides a slightly better field of view than the 70-300mm. The VR comes in VERY useful in lower light conditions (it never replaces a tripod in low light though) and at the tele end. On top of that I can attest to the quality of the new 55-200VR lens, a good buy! For the 18-70, no need to say, it is a better lens than the 18-55 overall.
Price wise, both will add up to around $700 (2nd hand 18-70 and a brand new 55-200VR) The only disadvantage is the lack of the 200-300 range, which for me is not a hassle cause i hardly use that range.
For tamron...I've tried a 18-200 lens before but sold it off in a short notice due to the noticeable (and very obvious) image quality. It is no where as sharp as my kit lens, the picture look flat and the auto focusing is a pain! However for the price, it is undoubtly a good lens for the budget user especially with the range.
Hope this helps!