I really feel for you there Eric, you seem to be one of many who are in a world of hurt with Vista conflicts.
I don't have Vista (XP user) so I'm not qualified to give any helpful advice or solutions for you. It could just be driver conflicts for your modem, and may just be a case of installing up-to-date drivers.
I don't know why it should affect your font sizes in your browser though, there must be something else going on there at the same time.
Hopefully, another Forum member who is a Vista user may be able to shed some light on your problems.
As you say, Vista is designed to work with Broadband, which is now the most popular method for Internet connection and ISP providers are now very competitive in the various packages on offer.
I'm sure you could find a suitable Broadband provider at a reasonable cost. The added bonus is that it will free up your phone line, and you'll get better connections on the Internet for general surfing and file downloads, and it should sort out your current problems with your modem.
The choice is a personal one and is decided upon how much use you wish to get out of the Internet.
Let's hope someone else can come up with a fix for you in the meantime.
Laurie.