"Vivarium" (plural "vivaria") is the name normally given the enclosures in which herps are kept. Just as an aquarium is solely a water habitat, a terrarium is solely a land habitat. For many herps a mix of both is required so the broader term which covers both extremes and everything in between is more appropriate.
Whatever the herp you are keeping the first consideration must be where and how you are going to keep it. And that will have to be in some form of vivarium. To keep the animal(s) happy the vivarium must set out to emulate the animals natural habitat. This involves many factors such as what sub-strate to use, the provision of hiding places, correct temperature gradients, lighting and, of course, the size.
Now, I can't and won't address all the possibilities but will instead discuss my vivaria and the rationale behind their design.
This is far and away the best approach when possible. I live on the South Coast of the UK and I can keep a surprising range of herps in such vivaria from most of Europe including even the hardier Mediterranean species. The vivaria range in size from around 20 sq metres to under 1 square metre. Sadly, one can't just put some sort of barrier around a piece of garden and say "That will do!". Some effort still has to be put into making the habitat contained suitable for the species to be kept in it. Other considerations include escape proofing and protection from predators. More details about my outdoor vivaria can be found here.
At its simplest, this is just a greenhouse or even a cold frame. In other words not only surrounded by some sort of barrier but roofed as well. Clearly it is important that light not be restricted. Again, I have a range of these, totalling about 25 sq metres. All of the factors under Outdoor Vivaria have to be taken into account but in addition the materials used become more critical, particularly in respect of light transmission. Properly done, however, the frost protection and increased temperatures allow many Mediterranean and Near and Middle Eastern species to be kept. Conversely prevention of overheating has also to be addressed. Once again, follow this link for more details.
Obviously with sub-tropical and tropical species neither of the above will suffice. A higher temperature has to be maintained particularly in the winter months. Indoor vivaria allow these factors to be controlled although this can be a costly process. My indoor vivaria range from conventional small (as little as 30 cms square for rearing some hatchlings) vivaria to large walk in vivaria of about 6 cu metres. Some are totally isolated from external factors while others receive the benefit of direct sunlight. Once again, the biggest problem to be addressed with the latter vivaria was to prevent overheating. More details about these various types can be seen here.
The things almost all of these vivaria have in common, with only the exception of the very small baby rearing ones, are that they set out to emulate natural habitat and have all had many different species breeding in them - and breeding repeatedly, often over several generations.