Prepare the crawfish boil:

Place water, Old Bay, fish sauce, lemongrass, lemons, garlic, and ¼ cup of the salt in a large stockpot.
Cover and bring to a boil over high.
Reduce heat to medium-low, and simmer 15 minutes.
Add potatoes, and cook until just tender, 15 to 20 minutes.
Add remaining ½ cup salt.
Increase heat to high, and bring to a rolling boil.
Add crawfish and corn; boil until crawfish are bright red, about 8 minutes.
Remove from heat, and let stand 15 minutes.

Prepare the butter sauce:

Heat ¼ cup butter in a stainless steel saucepan over medium-high heat until foamy.
Add lemongrass; sauté 4 to 5 minutes or until very soft, stirring very frequently.
Add remaining butter, garlic, salt, and cayenne.
Cook until butter is melted and garlic is just softened, 8 to 10 minutes.
Remove from heat; stir in lemon juice.
Cover to keep warm.

Finish:

Drain crawfish mixture.
Discard garlic, lemons, and lemongrass.
Place crawfish, corn, and potatoes in a huge bowl.
Toss with half of the butter sauce and cilantro; serve with remaining sauce.