What is the right software to design T shirts

Wow thats a loaded question. You are right that vector has the advantage of being able to scale up or down with no loss in resolution or detail. It also has other advantages (files size, sharp crisp lines etc...) However that doesn't necessarily make it the best tool for every application. For photorealistic effects and subtle gradients bitmap programs are superior. The particular program you use is really just a matter of prefference. You will get as many different opinions as there are programs. Here are my two cents on the subject. Adobe products are the industry standard. They are the programs of choice in most major graphics related businesses. They are very powerful and have all the bells and whistles. This fact also makes them relatively difficult to learn. There are also free programs that are pretty good and do and adequate job for most appliations. However, These programs are not widely used in commercial envioronments so if you ever intend to work or even exchange files or tips or info with professionals I recomend you don't learn on these because this will make the transition to commercial programs more difficult. Corel Draw is found in most sign and screenprinting shops. Usually along with Illustrator and photoshop. I would recomend you download the the free trials and play around with them untill you find the one you are more comfortable with. If money is an issue corel is cheaper. Goodluck