I don't know why a USN version wouldn't have a full length VLS installed, other than as intentional crippleware. As for the design preference, I like the Patrol Frigate variant of the Legend class cutter.
The Legends were built to commercial standards, which is one chief problem with that design. The militarized version hasn't given much details, i'd image it's classified to quite extent. The Class had alot problems, structural issues which had to be straighten out, there was communication suite defects, but that stuff won't be in the Frigate variant their trying to pitch. The Frigate will be larger, but there still questions about how good it would be. I think the Navy wants existing design that been thought it, verses a something only been used for police actions not blue water military operations such. The militarized version (patrol frigate variant) of the Legend doesn't even exist but on paper.
On different vein, apparently Spain is replacing their old Santa Maria-Class (Oliver Perry variant) FFGs with looks like a lesser version of the F-100 Class Aegis Frigates,
the F-110. These ships aren't using Aegis, they're going only have 16-Cell VLS launchers in them verses.