This hasn't quite been made explicit, so here's the principle.
Games last for an average of X turns (where X is some unknown variable which is probably less than 30). This means that even with the smallest deck allowed you will not be drawing all your cards unless you have some powerful card draw like Hourglasses or Sundials.
Before you start you need to work out what proportion of pillars you want in your deck (this can be modified by the card you put in, so you may want to revisit this step whilst tweaking your decklist). This then gives you a proportion of non-pillar cards that you should stick to.
A deck revolves around a few key cards. In your example they are:
Key Card 1: Minor Vampire - Low cost
Key Card 2: Blessing - Low cost
Key Card 3: Nightfall - High cost
Now, you want to draw as many Minor Vampires and Blessings as you possibly can, because they are what is making you win. Therefore, you put the maximum number of those in. This sets up the general principle which is that any card you add to the deck from this point onwards reduces the chance of you drawing one of these cards. Therefore, you include the absolute minimum number of other cards you can. Generally this means you want to have 30 cards in your deck in order to maximise your chances (one in five) of drawing a key card.
Nightfall is slightly different because multiple Nightfalls don't help. Ideally you want to draw one or two over the course of the game. You therefore work out the odds of drawing a Nightfall by turn X (or more likely before X if they are essential) with various different percentages and put them in the deck in that proportion. This principle can be used to work out how many supporting cards you want to include:
Extra Card 1: Drain Life- Low cost
Extra Card 2: Steal - Low cost
Extra Card 3: Dusk Mantle - High cost
Extra Card 4: Morning Star - High cost
You might want to draw one or two Dusk Mantles and Morning Stars over the course of the game, and maybe one or two Steals and Drain Lifes depending on how essential you feel they are, so you do the same thing. If there are any remaining slots that can't be filled with more key cards, then now is the time to add in those extra Drain Lifes. But you shouldn't just add cards above the total of thirty for the sake of it, since that way you're diluting your key cards.
Does that make sense?
