Should I be creating content using posts or pages?
Please see http://www.wpbeginner.com/beginners-guide/what-is-the-difference-between-posts-vs-pages-in-wordpress/ for more information on the difference between using posts/pages
Should I be using Visual Composer or another page builder on my site?
If possible, I would lean towards not using Visual Composer if there is already another page builder that shipped with your theme. As examples, X theme, Swift themes, and ElegantThemes typically come pre-packaged with their own builders, and when you try to introduce another page builder (such as Visual Composer), you may find that parts of pages become broken or have strange markup (ie, shortcodes) appear in your posts/pages. If you don’t know whether your skin came with its own page builder, then talk to your resident web developer. If you still prefer to use Visual Composer, then, as a rule of thumb, I would only use it when the builder that ships with your skin doesn’t offer the feature that you need. For example, if you are using the Divi page builder, there’s no reason to use Visual Composer to create a Tabs module, given that Divi already has its own tabs feature.