i ahve a problem where my shoe trees seem to be too small (in terms of their thickness, i guess) so there is not really much support to keep the vamp tight (?) not sure if i'm describing properly, but if anyone understands what i mean, is there anything i can do to remedy this issue?
If the trees are the recommended size for the shoe, then I would suggest adding an inexpensive rubber insole insert (the kind you can buy off the rack at almost any store) and use them with the trees to push the trees up against the vamp. You may need to be careful when doing this with split toed shoe trees as you could force a crease in the vamp over time.
Another option, if you are using solid toed shoe trees, is to use a tongue pad (can be bought online or at a shoe repair store). A tongue pad is a soft rectangular pad with a peel off sticky side you would normally stick to the underside of the tongue of your shoe to make it fit better with a low instep. But you could stick the tongue pad to the top of the shoe tree instead to achieve the tighter tree fit you are looking for.