Australian Daniel Ricciardo, who has outqualified his German team mate in three of the four races so far, is expected to have a new chassis some time in July.
"Drivers don't always want to change them - they can get attached to a particular chassis and when they are on a good run they like to hang on to it for as long as possible," said Marshall.
Vettel has had quite the opposite, even allowing for problems with the new Renault V6 engine which has been outperformed by Mercedes.
With just one podium appearance the German is enduring his worst start to a season since he joined Red Bull in 2009 and he has been asked to move aside for Ricciardo in the last two races.
The 26-year-old won the last nine races of 2013 on his way to a fourth title in a row.
Team principal Christian Horner said last month that the champion was very sensitive to the car's handling, and how it entered the corners, and was still looking for "the feel that he needs from the car".
Vettel won in Spain in 2011 and finished fourth in Barcelona last year.
(Reporting by Alan Baldwin, editing by Justin Palmer)
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