If you want to go the Mexican route, I assume some corn is necessary. In my experience, the sweetness from the corn can accent munich malt in an adverse way. Mine straddles the line of being almost too sweet and malty but it finishes at 1.011.
I brewed a Mexican-style Vienna this spring, and I did notice the toastiness of the Vienna and sweet corn flavor were initially quite prominant and at odds with each other. It took a couple of weeks on gas for those flavors to mellow out, but when they did I ended up with one of my favorite beers I've brewed in a long time.
Here's my recipe, FWIW. I didn't get the chance to order WLP940, so I used S-189. It is on the hoppy side (flavor-wise) for a Vienna, so you may want to skip the flameout addition if that's not what you're going for. I do think the higher IBU's help balance the corn, so you might want to leave the boil addition as-is.
HOME BREW RECIPE:
Title: Negra de Cinco
Brew Method: BIAB
Style Name: Vienna Lager
Boil Time: 60 min
Batch Size: 3 gallons (fermentor volume)
Boil Size: 4 gallons
Boil Gravity: 1.039
Efficiency: 80% (brew house)
STATS:
Original Gravity: 1.051
Final Gravity: 1.012
ABV (standard): 5.15%
IBU (tinseth): 24.93
SRM (morey): 14.94
FERMENTABLES:
2.5 lb - German - Vienna (48.8%)
1 lb - Flaked Corn (19.5%)
1.25 lb - German - Pilsner (24.4%)
4 oz - Belgian - CaraVienne (4.9%)
2 oz - American - Midnight Wheat Malt (2.4%)
HOPS:
0.5 oz - Sterling, Type: Pellet, AA: 7.1, Use: Boil for 60 min, IBU: 24.93
1 oz - Sterling, Type: Pellet, AA: 7.1, Use: Boil for 0 min
MASH GUIDELINES:
1) Infusion, Temp: 148 F, Time: 60 min, Amount: 12 qt, Sacc Rest
2) Infusion, Temp: 163 F, Time: 30 min, Amount: 5 qt, Alpha
YEAST:
White Labs - Mexican Lager Yeast WLP940
NOTES:
Adjusted color to dark amber with Brewer's Caramel (not Midnight Wheat)