this is a problem with your wireless modem it does not support on vista. by the way have you upgraded vista to sp1. There are lots of other users that are having the same problem with these types of modems. Well you can try the following out.
Changes that may help to increase the compatibility of Vista with older networking devices:
Disable the IP Helper service:
1. Go to Start and type in "services.msc" (without the quotes) and press Enter
2. Scroll down to the IP Helper service, right click on it and select Properties
3. In the dropdown box that says "Automatic" or "Manual", set it to Disabled and then click on "Apply"
4. Then click on "Stop" to stop the service from running in the current session
5. Click OK to exit the dialog
1. Go to Start and type in "ncpa.cpl" (without the quotes) and press Enter
2. Right click on each network connection and select "Properties"
3. Remove the checkmark from the box next to "Internet Protocol Version 6 (TCP/IPv6)
4. Click OK to exit the dialog
NOTE: You should do this for each network connection.
Disable the DHCP Broadcast Flag:
Link: Windows Vista cannot obtain an IP address from certain routers or from certain non-Microsoft DHCP servers
Go to Start and type in regedit and press Enter.
Locate and then click the following registry subkey:
In this registry path, click the (GUID) subkey to be updated.
If the key DhcpConnForceBroadcastFlag does not exist, use the Edit menu, point to New, and then click DWORD (32-bit) Value. In the New Value #1 box, type DhcpConnForceBroadcastFlag, and then press ENTER. If the key exists, skip this step.
Right-click DhcpConnForceBroadcastFlag, and then click Modify.
In the Value data box, type 0, and then click OK.
Close Registry Editor.
NOTE: You should do this for each and every GUID subkey.
Also please check the following link Windows Vista cannot obtain an IP address from certain routers or from certain non-Microsoft DHCP servers
Hope this Helps