The D3 level is what is important. Humans should only take D3 because it is what humans normally make.
Now since you started your question with “my Mother has positional vertigo”, I can make a suggestion but it is up to you and your Mom. She has a level over 40ng/ml which is important. We usually recommend a level of 50ng/ml but she is really close and hard to argue with that. If she took the 2000iu for a minimum 8 weeks before being tested, then that level will or should not rise any further.
If she doubles the amount she takes to 4000iu or even take 5000iu, theoretically her level will go higher in the range. The point to this is will a “higher” level help with her positional vertigo. It may but I cannot promise. The worse case scenario is if she tries it for say 4 months(only a guess) and sees no improvement, then just go back to taking the 2000iu.
The only other thing that can happen at anytime when taking Vitamin D, is it can cause mineral deficiencies and sometimes vitamin deficiencies. So if she were to take an increased amount and add any new symptoms or any old symptoms get worse then you have to come back and tell me so I can try and help you figure out what it is. Also if she has any other health issues for which she takes medications, keep an eye on that. Here is an example. Vitamin D is a natural blood thinner so if she took Coumadin on a regular basis it will change INR numbers and medication needs to be adjusted.
If you have more question, just ask!
I also always recommend reading http://www.vitamindcouncil.org/about-vitamin-d/vitamin-d-and-other-vitamins-and-minerals/.