The Whites sit 16th in the Premier League standings ahead of a second meeting with the Hammers in a week at the London Stadium.
Leeds have won four games so far in the top flight amid adversity and have given themselves an eight-point cushion over the relegation places.
The Elland Road outfit travel to the capital once again on Sunday to meet David Moyes' side, who came out on top 2-0 last weekend in a controversial third-round FA Cup clash.
Vlašić - who was involved in the VAR controversy last week which led to Manuel Lanzini's opener - believes it will be a hard game for both teams.
“I’m a little bit surprised [Leeds have struggled this season], but they are still a good team,” Vlašić said ahead of kick-off.
“We played them in the FA Cup and it was a hard game because they play a unique style of football, man on man all over the pitch, so it’s really hard to play against them, but it’s also really hard for them to play every game like that so we’ll see what happens at the weekend.
“Of course, last weekend’s game against them helps us. We analysed that game and saw what we did good and what we can improve.”
West Ham toppled Norwich City in midweek and are looking to make it three wins from three in seven days.
“We feel like we are back on track,” Vlašić added. “We had a few problems with results, but we are now back to where we were at the start of the season, so everybody in the team is really confident now and feeling good.”