Marko admitted to Germany's Bild newspaper: "If our disastrous state does not change soon, I could not blame him for thinking about a change."
Vettel, 26, sat down with his mechanics in Melbourne on Tuesday for his traditional pre-season dinner, where he named his uncompetitive RB10 car an unspectacular 'Suzie'.
Marko told Sport Bild magazine: "After the test in Bahrain, we would be happy if we finish in the points in Australia.
"We know that we have a good car," he added, "but we'll only know if the engine is good if we get it to work properly.
"The decisive factor is the new software that our engineers wrote for Renault."
The undoubted favourite for the Albert Park opener is Mercedes, and F1 chief executive Bernie Ecclestone on Tuesday tipped Lewis Hamilton to win the title.
But 1996 world champion Damon Hill told the Daily Mail: "I would warn Lewis not to underestimate Nico (Rosberg)."
Indeed, the flamboyant 2008 world champion Hamilton is often the Mercedes driver on everybody's lips, but the German told the DPA news agency that he doesn't mind.
"I do feel that the fans appreciate me," Rosberg said, "and I am grateful for their support."
Team chairman Niki Lauda said opinions about Mercedes' drivers are often formed because of Hamilton's "brutal force of talent".
But he told the Austrian broadcaster ORF: "Nico is characterised in that he thinks in a very technical way -- almost like Vettel.
"The two (Hamilton and Rosberg) complement each other very well."
Rosberg said: "I have a feeling that this could be our year and that's very exciting."
Still, the 28-year-old and the Brackley based team are cautious. "To say now that we will become world champions because of the good tests would be totally wrong and premature," boss Toto Wolff told the Stuttgarter Zeitung newspaper.
And a forecast of rain for Melbourne is making Lauda nervous.
"We have never driven with these turbo engines in the rain," he said, "so it is possible that even more problems could turn up."