Aberdeen-born Stuart Holden also arrived on a short-term deal after cutting his ties with Houston Dynamo.
All three players look set to be included in the squad for the vital home match against Fulham on Saturday, although Holden has been troubled by a thigh problem.
Coyle has thrown down the gauntlet to the trio as he attempts to drive the club up the table.
He said: "Vladimir and Jack were not going to start in their respective teams but they have an opportunity to do so here.
"It they get that chance it is up to them to hold on to the jersey. It is a fair fight for everyone at this club.
"If you are doing well you will keep the jersey. They are young exciting players. Stuart also has a chance."
It is a busy spell for Bolton, who go into the match after losing to Liverpool at the weekend.
Lying just a point above the drop zone they face ambitious Manchester City at Eastlands on Tuesday.
However midfielder Matt Taylor believes Bolton can still claim a place in the top half of the table.
"With the players we have, we are capable of doing that but we need to start picking up results," he told the club's website.
"Games are being crammed in now because of the bad weather and it is very demanding. But that is what we are all here for.
"I won't complain at two games a week because there is no better feeling than running out on the pitch."
Coyle could recall Swedish striker Johan Elmander as Ivan Klasnic remains sidelined with an ankle injury.
The Bolton boss might feel the need to beef up his attacking options especially on their own patch after promising to give supporters value for money.
He said: "We recognise when you have home backing and people work hard all week to support you, you want to give them something to shout about.
"It's an exciting month but one we are looking forward to. When we are at home we have to use that to our advantage."
Under experienced manager Roy Hodgson, Fulham got back on track in midweek by beating Portsmouth.
Coyle added: "They will have been lifted by that result. We know they are a very good side, run by a top-class manager.
"We are at home, however, and have to try to use that to our advantage. I believe we have a group of players who can win us crucial points."