Ultimately, we can't tell you exactly how long you can leave butter out. We have no problem with leaving butter out to soften as long as it is well-covered. It usually doesn't take us more than a couple days to go through a stick anyway! We've never had rancid butter in that time.
For complete instructions, see my weekly column. Note: The vinyl melts in just a few minutes and is warm to the touch when you pull it from the 200-degree oven (don't use a hotter oven or the vinyl will start releasing toxins). You can form it by hand immediately, but use leather gloves if you're nervous. All beginners in Record Upcycling should try a bowl.
So I wouldn't consider heating plastic. It will melt if the heat is to great. You will likely not heat your terrarium hot enough to melt most rigid plastics if you have plants inside. Whether it will melt depends upon which plastic your terrarium is made of and what temperature you are heating it to. A lamp heater posed some distance above the terrarium should not increase the temperature beyond what is comfortable to touch.