A sideboard is a set of up to 15 cards you can use between games to modify your deck. The assumption here is that there are some very useful cards that are specific to certain situations. They're specific enough to solve particular problems effectively, but too specific to keep in your main deck all the time. These situations are usually about strategies, but sometimes target color as well. You generally need to understand the weaknesses of you deck and the strengths you're likely to encounter, as opposed to blindly choosing cards that attack one color or another. Most importantly, you have to know when to add in sideboard cards and which cards in your main deck to swap out.
That said, I'm completely unqualified to build my own sideboard because I have only a hint of my own weaknesses and almost no knowledge of current decks other than my own. For now, my sideboard is a kind of first aid kit. I don't know what kind of injury I'm likely to sustain, but I've assembled the most likely remedies based on what other people do in emergencies.
I'm building my initial sideboard by using a deck database, looking up vampire decks that win tournaments. I looked at all their sideboards, and ranked cards by popularity. This provided me a blind list of what might be helpful, kind of like throwing together a bunch of different sized bandages in hopes your injury will be applicable. Some of the cards were already in my main deck (a potential warning sign). Here's what I came up with:
Popular Vampire Deck Sideboard Cards (in order of popularity)
- Mind Sludge
- Sadistic Sacrament
- Malakar Bloodwitch
- Marsh Casualties
I'm not sure how many of each I'll put in my sideboard for tomorrow (some are hard to find, like Sadistic Sacrament and Marsh Casualties). I actually think I'm far better off not running a sideboard or running less than fifteen cards if I don't understand how to use them. For that reason, I plan to be careful and only use my sideboard cards if I can identify a) what I'm doing wrong and b) how the sideboard card can solve the problem and c) which cards I can remove from my main deck without exacerbating the problem. Another hint from the article below is not to mess with you sideboard if you're winning. Good advice.
See Learn How To Sideboard, Dammit!
for sideboard advice (and some abuse).
Deathmark appears to be the most popular vampire deck sideboard card.
Cheap. Targeted. Useful 40% of the time.