In order to change the text alignment, open your vBulletin admin panel-> Styles & Templates -> Style Manager -> Edit Templates -> Go -> CSS Templates -> postbit.css.

Find the .postcontent declaration and enter the following line in it:

text-align: justify;

The declaration will be similar to the following one:

.postcontent {
text-align: justify;
word-wrap: break-word;
}

This way you can modify your posts' text alignment.