That's the number you need to remember, Liz. Overall length only effects the internal volume of the dulcimer. The VSL is important for picking the right string to tune and play the notes you need (called tunings).
A 28" VSL dulcimer can, generally speaking, be either a baritone or a 'regular' dulcimer depending on the gauges (diameters) of the strings you put on it, and the notes to which you tune the strings.
I strongly suggest you read the article-blog I wrote several years, aimed at folks like yourself, called I Just Got A Dulcimer, Now What? It's an illustrated glossary of dulcimer terms so when you ask questions you can use terms we all understand; plus answers to many beginner questions about the tuning, playing, care and feeding of your new friend.
The article is here: http://mountaindulcimer.ning.com/profiles/blogs/i-just-got-a-dulcim...
IMHO as a player for nearly 40 years, starting out with a "regular" dulcimer rather than a baritone would be to your advantage. To tune your 28" VSL dulcimer to 'regular' dulcimer tunings like DAA and DAd, you will want strings of the following gauges: bass string = 20 wound; middle drone = 12 plain steel; melody string (or 2) = 10 plain steel. Many acoustic music shops sell sets of 4 strings, any of which will do fine for getting started.
If you have access to a micrometer, you can measure the diameters of the strings on the dulcimer. If they are significantly larger in diameter than the numbers I gave below, then your dulcimer has been rigged as a baritone.
The number of soundholes is not what matters; it's the total area of the soundholes. Whether yours has one or two holes per bout, the builder has no doubt given enough hole area to let the sound out appropriately. There are other factors that also affect the volume of the dulcimer such as how hard you strum, how large the interior volume is, and many more. Basically, don't worry about it!
I see that you're a "Panhandler" from Milton, FL. If you ever come down coast to Fort Myers, give me a call and we can meet up and play some tunes.