Expansion teams in US sports always struggle in their initial seasons based upon how the initial squad for an expansion team is "drafted". All the other teams have to give up some of their squads, which the expansion team can chose from, however the existing teams can protect a certain number of players (11 in this case), preventing the expansion team from picking them. This means that, by default, the only players available to the expansion team are those which the other sides don't believe are their best players.
Imagine if the same was done in the Premier League, each existing club could protect 11 players, and you could only draft a maximum of 2 players from any given team, how piss poor would your squad be, the worst 50% of all the squads to chose your entire squad from! And it's worse in the MLS as the depth of talent is pretty terrible in most squads so, after you've protected 11 players, those ranked 12th or below will be really pretty poor indeed. Adding a selection of over the hill 35+ year olds who are just after some quick cash isn't going to help you too much either.
Vieira won't be an instant success as his ability to revamp the squad is hampered by the MLS's "unique" set of rules, he can't just go out and spend £40m on a new group of players, he has to deal in college drafts and player trades as well as a small amount of player purchasing. Where he should be measured is the tactical side of things, if he can get them more organised, passing the ball better, defending better then he'll have been successful. He should be looking to progress to the play offs (it is easier to make the playoffs in MLS than not, as only 4 sides out of the 10 in NYCFC's conference will fail to make them) and then see where that takes him.