Ingreedients look ok
Pitched a 2-liter starter of WY3711. ok
Fermented two weeks in primary (got up to 76F), + one more in the keg before cooling and carbonating. Fermentation behaved normally.
Fermentation: While, based on your FG everything looks good but how did you determine fermentation was finished? Both Acetaldehyde and Diacetyl are fermentation byproducts, normally from premature removal from premature removal from the yeast cake though they can be the byproduct of contamination.
You do indicate that you likely sensed the Diacetyl, "semi-slick mouthfeel" is how a lot of people sense Diacetyl.
Aside from my less than traditional hop bill, anything look suspicious? For such a low FG, the beer does not finish particularly dry with a semi-slick mouthfeel which is a little surprising. Thinking hard about my ingredients, it occurred to me my Willamettes are I believe from the '07 crop - old hops? Anything else look particularly Diacetyl or Vegetal producing?
The Old hops may contribute to the vegetal, though this too may be a byproduct of fermentation.
This compound is responsible for an artificial butter, butterscotch or toffee-like aroma and taste. At low levels, it may also produce a slickness on the palate. A significant number of tasters cannot perceive diacetyl at any concentration, so every judge should be aware of his or her limitations. Diacetyl is a fermentation by-product which is normally absorbed by the yeast and reduced to more innocuous diols. High levels can result from prematurely separating the beer from the yeast or by exposure to oxygen during the fermentation. Low FAN levels or mutation may also inhibit the ability of yeast to reduce diacetyl. Note that high fermentation temperatures promote both the formation and elimination of diacetyl, but the latter is more effective. For that reason, lager breweries often employ a diacetyl rest, which involves holding the beer in the 50-55 F range for a few days after racking to the conditioning tank. Diacetyl is also produced by some strains of lactic acid bacteria, notably Pediococcus damnosus. Low levels of diacetyl are permissible in nearly all ales, particularly those brewed in the United Kingdom, and even some lagers, notably Czech pilseners.
This compound has the taste and aroma of fresh-cut green apples, and has also been compared to grass, green leaves and latex paint. It is normally reduced to ethanol by yeast during the secondary fermentation, but oxidation of the finished beer may reverse this process, converting ethanol to acetaldehyde. Elevated levels are generally present in green beer or if the beer is prematurely removed from the yeast. It can also be a product of bacterial spoilage by Zymomonas or Acetobacter. Background levels of acetaldehyde can be tasted in Budweiser due to the use of beechwood chips to drop the yeast before it can be reduced to ethanol.
This is the flavor and aroma of freshly cut grass or green leaves. Responsible compounds include the aldehydes hexanal and heptanal, which are produced by the oxidation of alcohols in the finished beer or the deterioration of improperly stored malt or hops. Some English and American hop varieties produce grassy notes if used in large quantities, but this flavor should not be a significant part of the profile.
Vegetal is often DMS
DMS, or dimethyl-sulfide produces the aroma and taste of cooked vegetables, notably corn, celery, cabbage or parsnips. In extreme cases, it may even be reminiscent of shellfish or water in which shrimp has been boiled. DMS is normally produced by the heat-induced conversion of S-methyl-methionine (SMM), but most of this evaporates during a long, open, rolling boil. A short, weak or closed boil, or slow cooling of the wort may therefore lead to abnormally high levels. Some DMS is also scrubbed out during a vigorous fermentation, which is why lagers and cold-conditioned ales may have slightly higher levels than warm-fermented ales. Wild yeast or Zymomonas bacteria may produce high enough levels of DMS to make the beer undrinkable. Pilsner malt contains as much as 8 times the SMM of pale malt, so Pils-based beers sometimes have a DMS character; this is a much more common cause in most beer than a covered boil. Low levels of DMS are appropriate in most Pils-based lagers, particularly American light lagers and Classic American Pilsners, but are not desirable in most ale styles (Cream Ale is a notable exception).