Yes it is a myth. There have been posts about testosterone as well as testosterone and soy before. I suggest you search on "soy", "testosterone" and "soy and testosterone" with the search link on the top right corner of this site. Also, repeat those searches onhttp://www.veganfitness.net/index.php
You should find at least several posts of weight lifters who had their testosterone levels measured and who had no problems.
FWIW, if you have health insurance it is worth the price of a copay for one visit to a medical doctor to easily, quickly lay any concerns you have to rest. It will be less of a hassle and give you a better answer than sorting through the folklore on the internet
As far as soy goes I collect articles about it in a special section of my site. It provides a balanced view to the anti-soy hysteria you so often see on the web and all of the articles point to reliable authorities:http://beforewisdom.com/blog/category/soy/
The biggest issues weight lifters face when going vegan are getting enough calories, getting enough protein, getting a b-12 supplment and taking a DHA/EPA ( essential fatty acid ) supplement.