There's a trick to getting a new vac pot filter on. First thing is to get some replacement cloth filters.
Once you remove the old cloth, boil some water to rinse the new cloth through. A trick Ben C. showed me is that you have to start with a warmed and wet cloth filter and stretch it tightly when tying it on. Taking a little time to rinse it with water and then stretch it out making certain there are no folds or areas of crimping at the edges before tying it tight is essential.
The reasoning for this is largely that the filter needs to be tight but also that the filter needs to set flush in position against the glass when brewing. Any crevices or uneven spots around the edge will allow steam bubbles to gurgle from the sides of the filter. This is uncontrolled agitation and will add unpleasant bitterness (not unlike whisking the grinds excessively during brewing). Make sure that filter sits flush when brewing.