Can someone explain bra measuring?

No never add inches to the under bust! If your under bust measurement is 28 then your band size is 28. Then to get cup size you calculate the difference between your bust and underbust measurements. 33-28=5. That would make you a 28DD because AA is .5" difference, A is 1", B is 2", C is 3", D is 4", DD is 5" etc. to get a more comprehensive and accurate measurement go on http://www.brasizecalculator.tk

If you find that 28 bands are too tight feel free to try on 30Ds. That is the sister size of 28DD. I wouldn't recommend buying bras with bands any larger than that though.

That sizing method you updated with makes absolutely no sense at all (over bust measurements mean nothing in bra sizing unless you wear your bras with the band over your boobs and the cups up on your neck.....), but go with whatever size you want, although you measure as a 28DD or 30D.

Your band size isn't just a random made up measurement based on over bust and adding inches... it's literally the length the band has to be to go around your ribcage which is your underbust measurement.

Both Takki and Adryana have very good information.

A girl sized as a 32DD would have about a ~32" underbust and a ~37" bust.

Update:
In response your your third update, you should measure the band stretched--that's what determined the band size. So for instance one of my 28D bras the band measures 21 inches when it is neutral and around 28-29 when stretched out. Actually, VS in particular is known for having stretchy bands (although I believe their new Incredible bra in 32 fits a bit more like a 30). Based on this I am pretty sure your bras are true 34Ds. A 34D and a 28D are quite different cup sizes because the cup size scales based on band size.

Again, wear whatever size you want and don't bother asking on Yahoo answers if you don't want a straightforward and truthful answer. Use the information provided by Takki, Adryana, and I if you'd like an accurate measurement for yourself.

Update:
As I have said before, you can of course go with whatever size you want. I personally find any band size larger than a 28 (and even the more stretchy 28s) very uncomfortable since my underbust is 26" snug and just under 25" tight. As a 28 you might feel more comfortable in 30 bands and perhaps the VS 32 you have is one of the new ones that fits more like a 30 which is why it's not as stretchy (my sister just tried out a number of their 32DDDs so I know that most of their bras are true 32s). I am not a fan of VS bras (or their fitting methods) so I don't know as much about them, all I know is that a true 34 band would (stretched out) be larger than your measurement around your boobs and therefore theoretically could ride up over your breast tissue. But if you like them then that's perfect!

Let me explain the last little bit that the other two girls (Takki and ?, who btw fully explained everything else to you very well) left out. When you wear a cups that's too small, the band will feel too small. That's because the band is being stretched to cover extra breast tissue. When you wear a band and cup that's the right size, you won't have this problem. You may have to do the leaning measurement (that's how I have to measure and it gives me about 2-3 more inches that correlate to my actual bra size) to account for all your breast tissue. For example...I wear a 32J/JJ...I measure, standing straight and not leaning over, a 34H. That's about 2-3 cup sizes larger and a band size smaller than my standing measurement. Also, always wore 38 bands because I was a chunky girl (180lbs) but once I got sized properly, wearing a smaller band is nothing. My bras fit great and feel great and I no longer feel like I'm just not made correctly because of bras not fitting like they're supposed to. Please remember that breast shape also plays a role in how a bra fits. Unless you have perfectly round, perky breasts, a leaning measurement is almost always required. You do in fact measure a 28DD but you may find that you have to wear a 28DDD/F instead. Try the size on before you determine it doesn't fit.

Here's a link that explains everything very well: http://obsessedwithbreasts.wordpress.com/how-to-determine-bra-size/
Measurements are best done braless.

...your overbust isn't your band size.

Your underbust is your band size.
It might feel too tight BECAUSE YOU'RE USED TO NASTY ILL-FITTING BRAS.

"?" is completely right in how you do your measurements.

You might need to do a "leaning" measurement in order to get a more accurate bust measurement. My leaning measurement adds about 1 to 2 inches to my bust and that's how I have my current bra size.

I think you may measure the wrong location. For a 32D bust, your under bust should be 32 and your bust size should be 7-8 inches larger, that is 39-40.

Take this picture as reference, although it is in centimeters instead of inches,
http://en.wikipedia.org/wiki/File:EN-13402-bra.png
In the picture, the bust is 33-34 inches and the underbust is 27-28, the difference is 6 inch, so it is B/C Cup
This is a table for look up the size
http://en.wikipedia.org/wiki/File:Cup-conversion.svg

For the add 5 method you use, I am not familiarized but it should have another table for lookup the bra size.