Yes but water vapour is relatively constant. In effect, we can more or less rule it out, though in a future warmer world there may well be higher levels of it in the atmosphere. Carbon dioxide levels are not constant - they have been rising steadily since the start of the industrial revolution. Your figure for methane is wildly out. In the last 250 years methane levels have increased from 700 parts per billion to 1850 parts per billion, an increase of about two and a half times. Molecule-for-molecule, methane is a far more potent greenhouse gas than carbon dioxide, which is potent enough. The threat to the planet is caused by these two gases increasing rapidly in the atmosphere. It behoves opponents of global warming theory to at least get their facts right.