Town drew a mouthwatering home clash against the former Premier League champions in the third round but they must first win at non-league Hereford if they are to welcome England forward Vardy back to Highbury.
A Bulls side four levels below Town held them 1-1 on Saturday and will host the replay next week.
Rosler says the draw is a great incentive for both clubs, especially as Town have never faced a Premier League side competitively, having lost to Championship opposition on their two previous appearances in the third round.
Vardy’s 31 goals in 2011-12 fired Town into the Football League and earned him a move to Leicester.
But Rosler stressed the need to focus on beating Hereford – one of four non-league teams left.
He said: “It is a mouth-watering draw, especially for us given the Jamie Vardy link and the fact we have never played a Premier League club.
“But we have to get past the first hurdle before we think about the second hurdle and Hereford is a big hurdle for us. We need to make sure we get the job done.”
Town are without a win in five games and Rosler admitted after last month’s 4-2 defeat at Walsall that this is the most challenging time since his arrival in summer 2016.
And he aims to return to winning ways tonight, when League Two side Chesterfield are the visitors at the first knockout stage of the Checkatrade Trophy.
Rosler said: “We are not used to not winning. Since I came we were winning most of the time. We had a very high win percentage.
“Now we have not won and obviously we have a very young squad, You see the confidence of some players is shaky at the moment.
“That momentum we have to turn around. As coaches we are responsible and I am responsible, but the players (are) as well. We must all turn it around.
“It is a great development time for our players. There will always be periods when things are going easy and going your way,
“There will also be periods when you can’t really see where the next three points are coming and you are grinding them out.
“If you stay focused and do the things you used to do, then things will turn your way again.”