Arsenal failed to get Vermaelen's red card overturned, and Ferguson said: 'It's so inconsistent, it's difficult to analyse why Rio's was frivolous and that wasn't.
'Only the FA can answer that. They don't come out with a report on it, do they? It's sitting in the chambers of their minds.'
Frivolous: Thomas Vermaelen appealed against his red card against West Ham but did not have his ban extended by the FA
The United manager also reacted sceptically to reports that Ryan Giggs might play for Wales again at the age of 36.
'You must be joking,' said Ferguson. 'I can't see it. It must have been a weak moment for Ryan!'
The United veteran holds 64 caps for Wales, but has not played for his country since quitting in 2007 to prolong his club career.
However, when asked if he would consider an international comeback for a Euro 2012 qualifying campaign that sees Wales face England, Giggs said: 'It's something I would have to talk about with (Wales manager) John Toshack and Sir Alex. But I wouldn't dismiss the idea.'
Dragon: Welsh wing-wizard Ryan Giggs has suggested recently that he could return to the international fold for the first time in three years
Giggs, the current PFA Footballer of the Year, later qualified his comments by saying he had 'no immediate plans for coming back' and would only consider doing so in an emergency.
Wales will be without the injured Aaron Ramsey for their opening Group G qualifiers against Montenegro, Bulgaria and Switzerland this year and the need for a creative midfielder in the side is believed to have prompted Giggs's change of heart.
However, Giggs may be more likely to return as Wales's manager than as a player. 'If you go into management the big job you want is to manage your country,' he said.
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