Is using a yeast starter 100% essential when brewing a lager or can you use a few liquid vials?
You can make good beer by using yeast directly out of whatever packaging style your favorite yeast comes in. You can always pitch more by buying more too.
However, IMO, better beer and great beer is made when using a starter. Its a function of viability and vitality of yeast. Pitching more packs increases the # of viable cells you pitch. However, a starter increases the number of viable cells and increases the vitality of those cells.
A simple experiment on the power of yeast vitality is to brew up a beer with your favorite packaged yeast and toss it right in without a starter. Even use two packs! Plan your next brew session for when the primary ends. Transfer your beer to secondary or bottling as you would have planned, then pitch a cup of the yeast slurry into your next beer. Essentially what you have there is a great pitch of yeast with super vitality. (of course the cell # thing will contribute too. But you can correct for that with the MrMalty calculator, although the results will be the same). You'll get a faster ferment and if you taste the two finished beers side by side BLIND; the repitch will be better in most cases.
Here is further discussion on the same viability and vitality matter.
The goal is to pitch an adequate number of viable yeast cells in order to get a good fermentation. Whether you do that with a starter you built yourself or by buying more yeast does not matter. A useful tool is Jamil "Mr Malty" Zainasheff's pitching rate calculator.
In the long run it's cheaper to buy yourself a 5 liter erlenmeyer flask and learn how to make a good starter. In a pinch though, buy more yeast.
I've brewed two 5 gallon batches of Lager, both times with dry lager yeast and both times without starters (the dry yeast package instructions said to sprinkle it into the wort, so that's what I did).
The first one turned out really well and the second one is currently bubbling merrily in my 40 degree basement.
I don't have any experience with liquid vials but in my experience it's not 100% essential to have a starter. However, I'm sure making a starter and pitching more yeast certainly couldn't hurt.
Always make a starter for all beers.
Use a growler if you have to.
Takes 15 minutes.
Since I have started making starters 2.5 years ago, I have not had one infection, I always know that my yeast is viable, I pitch near the correct amount of yeast, and my beer tastes better. No infections, and I brew once a week.
This way was suggested to me on a brewers guild mailing list I subscribe to:
you need to have a stir plate and a flask. the flask really can be anything as long as it has a small mouth to it.
put a smack-pack or vial of liquid yeast with two cups of water and 1/3 cup of dry malt extract. allow it to sit and stir for 24 hours and you'll see up to a 5x increase in cell count.
like I said at the beginning, this isn't my created way but one that was suggested to me. In my experience it works remarkably well. I'd strongly suggest it.