Is it possible to underpitch yeast that you know to be 100% viable??

Yes, because the pitching rate has to do with the OG of the beer and is directly correlated to the viability of the yeast.

For example, assume the viability is 100% and the pack contains 100B cells, but the beer you are brewing requires 300B cells for the correct pitching rate you still need a starter to get your 300B cells.

IF the yeast viability is 80% then you would need to grow 20% more cells to achieve the same starter for correct pitching rate.

Most liquid yeast packs will tell you that they are good to direct pitch up to 1.050, that assumes 100% viability and generally is still a little under the required pitching rate.

You can go to: http://yeastcalc.com, plug in the pertinent numbers and determine the required pitching rate for any beer you want to brew.

^Second that. It certainly helps if you are under pitching with 100% viable yeast but you are still under pitching. What's your recipe look like in terms of OG.

Just did this with white labs California ale.

just took a bit longer to show active signs,

^that's funny you mention this because I too just did this with a California Common although I used San Francisco lager yeast. I just moved mine today to a warmer temp to finish out fermenting and I'm not sure why but I've overshot the attenuation rate of the yeast. I'm at 75% already and it's still bubbling every now and then... Not sure if this is good or bad...guess we'll see.

^ Scratch that; though you said California lager. Although I guess it still applied. The main thing about under pitching is that when cells are reproducing they are leeching off flavors and ester's from that process so the more they need to multiply the more stress they are under and the more they have to multiply so the more the leech off. Every time a cell divides some stuff from inside of it leaks out which will give you the flavors of the yeast.