I have a similar USB to VGA device that uses sisusbvga and am now having a problem with it in 10.04. It was working perfectly in 9.10 and 9.04.

joel@disco2:~$ lsusb
Bus 004 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 003 Device 002: ID 04f2:0111 Chicony Electronics Co., Ltd KU-9908 Keyboard
Bus 003 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 002 Device 002: ID 09da:0006 A4 Tech Co., Ltd Optical Mouse WOP-35 / Trust 450L Optical Mouse
Bus 002 Device 001: ID 1d6b:0001 Linux Foundation 1.1 root hub
Bus 001 Device 004: ID 0711:0900 Magic Control Technology Corp. SVGA Adapter
Bus 001 Device 001: ID 1d6b:0002 Linux Foundation 2.0 root hub

So mine has a slightly different ID (compared to above entries) that did not need manually entered into the driver or anything like that to load.

What is happening is that it loads fine at start up and appears to be working great until I start to manipulate a window on the display connected to the USB->VGA device.  After doing this it will completely lock the machine up and freeze both displays while it shows the mouse cursor flickering and slightly moving back and forth.  This does not happen *everytime* I do something on the usb display but it is happening consistently enough to make it completely unusable.  When it happens I have to do a hard reboot.

dmesg:

[   37.756772] usb 1-4: USB2VGA dongle found at address 4
[   37.756894] usb 1-4: Allocated 8 output buffers
[   37.941613] usb 1-4: 8MB 1 ch/1 r SDR SDRAM, bus width 32
[   40.410259] usbcore: registered new interface driver sisusb

joel@disco2:/var/log$ cat Xorg.0.log | grep SISUSB
(**) |-->Screen "Screen[SISUSBVGA]" (1)
(**) |   |-->Monitor "Monitor[SISUSBVGA]"
(**) |   |-->Device "Device[SISUSBVGA]"
(II) SISUSB: driver for SiSUSB chipsets: SIS315E/PRO USB
(II) SISUSB(1): SiSUSB driver (2005/09/28-1, compiled for X.org 1.7.2.0)
(II) SISUSB(1): Copyright (C) 2001-2005 Thomas Winischhofer <email address hidden>
(II) SISUSB(1): *** See http://www.winischhofer.at/linuxsisusbvga.shtml
(II) SISUSB(1): *** for documentation and updates.
(--) SISUSB(1): Found USB dongle (device /dev/sisusbvga0, kernel driver 0.0.8)
(--) SISUSB(1): sisusbfb not found
(**) SISUSB(1): Depth 24, (--) framebuffer bpp 32
(==) SISUSB(1): RGB weight 888
(==) SISUSB(1): Default visual is TrueColor
(==) SISUSB(1): Device will be re-probed forever after disconnection
(==) SISUSB(1): Using HW cursor
(==) SISUSB(1): Color HW cursor is enabled
(==) SISUSB(1): SiSCtrl utility interface is disabled
(--) SISUSB(1): DRAM type: Single channel 1 rank SDR SDRAM
(--) SISUSB(1): Memory clock: 143.180 MHz
(--) SISUSB(1): DRAM bus width: 32 bit
(II) SISUSB(1): Using 7616K of framebuffer memory
(==) SISUSB(1): Using gamma correction (1.0, 1.0, 1.0)
(II) SISUSB(1): Gamma correction is enabled
(--) SISUSB(1): Memory bandwidth at 32 bpp is 143.18 MHz
(==) SISUSB(1): Min pixel clock is 10 MHz
(--) SISUSB(1): Max pixel clock is 119 MHz
(II) SISUSB(1): Replaced entire mode list with built-in modes
(II) SISUSB(1): Monitor[SISUSBVGA]: Using hsync range of 30.00-90.00 kHz
(II) SISUSB(1): Monitor[SISUSBVGA]: Using vrefresh value of 70.00 Hz
(II) SISUSB(1): Clock range:  10.00 to 119.32 MHz
(--) SISUSB(1): Virtual size is 1024x768 (pitch 1024)
(**) SISUSB(1): *Default mode "1024x768": 75.2 MHz, 56.6 kHz, 70.2 Hz
(II) SISUSB(1): Modeline "1024x768"x70.2   75.17  1024 1048 1184 1328  768 771 777 806 -hsync -vsync (56.6 kHz)
(==) SISUSB(1): DPI set to (96, 96)
(II) SISUSB(1): Setting standard mode 0x64
(II) SISUSB(1): Framebuffer from (0,0) to (1023,1902)
(==) SISUSB(1): Backing store disabled
(==) SISUSB(1): DPMS enabled
(II) SISUSB(1): Initialized SISCTRL extension version 0.1
(II) SISUSB(1): Registered screen 1 with SISCTRL extension version 0.1
(WW) Apr 30 03:18:52 NVIDIA:  - Screen 1: SISUSB