First, VGA is analog only and HDMI is digital only so this part of your request is impossible unless you use some digital converter box (probably 50-100$). If you search the web, you will probably find VGA to DVI adapters/cables and also DVI to HDMI adapters/cables, but don't be fooled, it won't work; the "fine prints" is that the first cable will be VGA to DVI-A and the second one will be DVI-D to HDMI. The difference being that DVI, unlike HDMI, can carry both digital (DVI-D) and analog signal (DVI-A); both combined being designated as DVI-I.
Second, when you say you want to use your HDTV as a second monitor, you want the second monitor to be a copy of what is shown on the first or you want the 2nd to "extend" the first? For the later, a simple "Y" cable won't work.
Depending on exactly what you are trying to accomplish, getting a new video card with both DVI-I and HDMI output could be a simpler and more elegant solution, such card can cost as little as 50$ (so less than a VGA to HDMI conversion box). One such card is the SAPPHIRE Radeon HD 4650 512MB.