Here is some example code. It is a group containing two commands. The first command sets the event timer for 8:00 pm (20:00). The second command is the TTS announcement (accompanied by an OSD.ShowText message for good measure).
If you want to try using this code yourself, select all the text in the code box below, copy it (ctrl-c), open your VC tree editor, right click, and select paste to put this group into the tree.
<?xml version="1.0" encoding="utf-16"?>
<commandGroup open="True" name="Reminder" enabled="True" prefix="" priority="0" requiredProcess="" description="">
<command id="443" name="set reminder time" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="This command is triggered by the event VC.Loaded (i.e., when VoxCommando is first launched and loaded, VC will cue the event timer called "TabletTime" to run at 20:00.">
<action>
<cmdType>VC.SetEventTimer</cmdType>
<cmdString>8:00 PM&&TabletTime</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<event>VC.Loaded</event>
</command>
<command id="438" name="TTS reminder" enabled="true" alwaysOn="False" confirm="False" requiredConfidence="0" loop="False" loopDelay="0" loopMax="0" description="This TTS reminder is triggered by the event called "TabletTime", which we defined in the "set reminder time" command. At the end we set a timer for 24 hours to call this command again, in case you leave VC on all the time, since the VC.Loaded event is only fired once. I'm not sure what will happen to the timers if your computer goes to sleep though.">
<action>
<cmdType>TTS.Speak</cmdType>
<cmdString>Excuse me. It is {ShortTime}. You asked me to remind you to take your meds.</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>OSD.ShowText</cmdType>
<cmdString>Excuse me. It is {ShortTime}. You asked me to remind you to take your meds.</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<action>
<cmdType>VC.SetEventTimer</cmdType>
<cmdString>23h 59m 59s&&TabletTime</cmdString>
<cmdRepeat>1</cmdRepeat>
</action>
<event>TabletTime</event>
</command>
</commandGroup>
edit: added a timer at the end of the reminder, that calls itself every 24 hours.