Borderlands – Is Vermivorous always level 50 (and up)

You need to find a Varkid larva, allow it to pod into an adult Varkid. Then allow this to pod transform into a Badass Varkid, wait until they form into a pod yet again, into a Super Badass Varkid. Let it pod transform once more into an Ultimate Badass Varkid.

After that it will have to kill other monsters, (like a Goliath would) and eventually it will form a pod, and it will become Vermivorous. Source (note it is a riflegaming vid, he has a type of humor that is not for everybody)

Unconfirmed: Some sources say that Varkids need other Varkids around for one to transform into a pod. Not sure about this one.

So TL;DR:

Larva -> Pod -> Adult -> Pod -> Badass -> Pod -> Super Badass -> Pod -> Ultimate Badass -> Let it go on a murder spree -> Pod -> Vermivorous.

In playthrough 1, outside of the DLC, there's no such thing as scaling. Enemy levels are determined solely by the area they're in and your mission (certain missions effectively "level up" areas, notably Sanctuary and The Dust). You can easily check the level of an area by checking out the vending machines in the area, they sell items scaled to the level.

DLC in playthrough 1 scales to your level...ish. It maxes out at level 30. I played the DLC as a level 34 character and was disappointed to see everything was at level 30 in playthrough 1. Not sure if it has a minimum level it scales to.

Playthrough 2 works the same way until you beat the Final Boss; then everything in that playthrough scales to your level (except mission rewards for missions picked up before the final boss).

Playthrough 3 works the same was as Playthrough 2 after the final boss: Everything scales to your level, period. In team play, it will always be scaled to the highest level player's level. And you have to be at least level 50 to unlock it, so enemies are effectively level 50-61 at all times. Though if you take a mission before leveling up, the mission will still stay at that lower level, as always.

Basically the game works like a standard RPG without level scaling until you A) beat the game twice (because you should be maxed out anyway) or B) enter a DLC (because you can do that at any time so a preset level would be a really bad idea)