I’m just a fellow user so I don’t know for sure, but I believe what you get with Vegas 17 Suite is basically the activation code, which you then use to activate SL6 after you’ve downloaded it from the Steinberg website, which would suggest it’s the full product and not bound to that version of Vegas. MAGIX (and Sony before them) had a nasty habit of bundling “sweeteners” with their “upgrades” which then stop working with the next “upgrade”, but in this case I think you get the same SL6 product you would have gotten had you bought it directly.
I put the word “upgrade” in quotation marks because the Vegas offers are not really upgrades, they’re new purchases of the latest version of at a discounted price. There are never bug fixes to previous versions so you have no choice but to buy it again. Pay close attention as well because I think SL6 is only incuded in the “Vegas 17 Suite”, which might not be the best value for your money, depending on what products you already own. I see it currently at US$349 for the upgrade.