Here's a bit of background information. Vista's Service Pack 2 was released in May 2009 and comes bundled with the old IE7 browser. If I remember correctly, the first Windows Update you run after SP2 is installed should upgrade your browser to the original build of IE9 (Update Version 9.0.1 / KB982861) from May 2011, and it usually takes one or two more Windows Updates after that to get IE9 fully patched to Update Version 9.0.57 / KB3203621.
I recall another Vista SP2 user having a similar error 9C48 when they tried to update the 32-bit version of IE9 in October 2016 - please see my post # 186 in the bleepingcomputer thread Windows Vista Update Hangs at "Checking For Updates". In Dredredre's case, she had a special "IC" (Internet Content Provider) build of IE9 that seemed to confuse Windows Update. We were eventually able to update her browser using the latest available standalone .msu installer, but Windows Update kept trying to offer the Optional update for IE9 dated 31-May-2011 that failed with an error 9C48 (see post # 188). She eventually hid that Optional update so that it wasn't re-offered every time Windows Update ran, and as far as I know her monthly cumulative updates for IE9 have continued to install normally every Patch Tuesday since then.
I'm not sure why your Update Version of IE9 is RTM (an early Release to Manufacturers version, first distributed March 2011) rather than the general v9.0.1 that most users with Vista SP2 and IE8 received via an automatic Windows Update in May 2011, but if the name of your Optional update is "Windows Internet Explorer 9 for Vista" (with no KB number), then Windows Update is likely trying to install the same May 2011 version of KB982861 that was being offered to Dredredre and is getting confused because you already have the RTM version of KB982861.
Let's try one more time and see if KB3203621 will install now that your Vista SP2 OS is patched. Go to http://www.catalog.update.microsoft.com/Search.aspx?q=KB3203621 Vista and download the 64-bit Vista SP2 installer. Save it to your desktop, close your browser, double-click the .msu installer to run, and then re-boot when prompted if the installation is successful.
32-bit Vista Home Premium SP2 * Firefox v51.0.1 * NIS v18.104.22.168 * MBAM Premium v2.2.1