Here's a new free Custom Command/Extension you can use with a sample XML file written by jetdv. Simply run the linked installer:

http://www.jetdv.com/scripts/TransitionsAB.exe

When you start VEGAS, it will now be under View - Extensions - Transition A/B (Free). Select an event on the timeline, choose the desired entry in the dropdown box, and click on "A" or "B" to add the necessary effects to transition out or in.

Instructions to modify the extension: Transitions A/B Extension – A Free Christmas Gift for Everyone – JETDV Scripts (http://www.jetdv.com/2021/12/24/transitions-a-b-extension-a-free-christmas-gift-for-everyone/)

https://youtu.be/7n2YlX2v0Y0

To modify what effects and keyframes are used, you simply need to modify the XML file located at [My Documents]\Vegas Application Extensions\TransitionsAB.config

Two critical lines:

<init name="NumberOfEffects" value="2" />
<init name="MaxEntriesInList" value="3" />

These must match the number of effects defined and the number of selections defined. The rest of the lines in that section are just "maximum" values so as long as you stay under the maximum you are fine. But those can be increased if needed.

Defining an effect: each effect is defined individually. You must specify the Name of the effect, the GUID of the effect (The GUID is actually used to add the effect so it MUST be there) and the preset to be used. I included the "num" after the "Effect" simply so you could identify this is effect number 0, the next is effect number 1. That way you won't get confused when setting up the selections but that number is not used anywhere.

<Effect num="0">
<Name>Gaussian Blur</Name>
<GUID>{Svfx:com.vegascreativesoftware:gaussianblur}</GUID>
<preset>Medium Blur</preset>

To find the GUID, you can use the instructions found in this tutorial: Reading Available Effects, Transitions, Generated Media, and Renderers in Vegas Pro – JETDV Scripts (http://www.jetdv.com/2021/06/21/reading-available-effects-transitions-generated-media-and-renderers-in-vegas-pro/)

Then you specify one or more parameters in that effect to be modified. You must specify the name of the parameter and then all of the keyframe information for that parameter. You can have up to 5 different parameters unless you increase the "Max" number at the top. I have defined the "A" Parameters to be at the end (transition out) and the "B" Parameters to be at the start (transition in) but it really doesn't matter how you define them. Only DOUBLE Parameters are allowed to be modified at this time!!!!

<Parameters>
<Parameter>
<PName>VerticalRange</PName>
<AParms>
<Aparm Location="Start" value="5.0" />
<Aparm Location="+15" value=".88" />
<Aparm Location="+60" value="0.0" />
</AParms>
<BParms>
<Bparm Location="Center" value="1.0" />
<Bparm Location="-3" value=".88" />
<Bparm Location="End" value=".4" />
</BParms>
</Parameter>
<Parameter>
<PName>HorizontalRange</PName>
<AParms>
<Aparm Location="Start" value="5.0" />
<Aparm Location="+10" value=".88" />
<Aparm Location="+30" value="0.0" />
</AParms>
<BParms>
<Bparm Location="-30" value="1.0" />
<Bparm Location="-10" value=".88" />
<Bparm Location="End" value=".4" />
</BParms>
</Parameter>
</Parameters>

Here's what the "locations" mean:

"End" place at the end of the event
"Center" place at the center of the event
"Start" place at the beginning of the event
+(timecode) place timecode distance (in frames) from the beginning of the event
-(timecode) place timecode distance (in frames) from the end of the event

Then you must set the "Selections". These are what show up in the dropdown box. This also specifies which of the above effects are used by that selection.

<Selection>
<Name>Gaussian Blur + Glow</Name>
<EffectsToUse>
<Uses>0</Uses>
<Uses>1</Uses>
</EffectsToUse>
</Selection>

This is a sample selection that uses both of the defined effects. I have it set to allow up to 10 effects but that can also be increased if needed. You just specify one or more "Uses" entries and use the number of the effect. Internally, the effects will be processed starting at the top and working downward. So the first one will be zero, the second one is one, third one is two, etc...

So you can easily change this to use whatever effects, parameters, and keyframes you want to use. Then you can define the selections that use one or more of those defined effects.

Per request there's a new version uploaded that allows changing the interpolation per keyframe.

The options you can add are: Fast, Hold, Linear, Sharp, Slow, Smooth, and Split.

<Parameter>
<PName>VerticalRange</PName>
<AParms>
<AParm Location="Center" value="1.0" Interpolation="Fast" />
<AParm Location="-3" value=".88" Interpolation="Slow" />
<AParm Location="End" value=".4" Interpolation="Smooth" />
</AParms>
<BParms>
<BParm Location="Start" value="5.0" Interpolation="Linear" />
<BParm Location="+15" value=".88" Interpolation="Sharp" />
<BParm Location="+60" value="0.0" Interpolation="Fast" />
</BParms>
</Parameter>

Also per request, I just uploaded another new version. In this version I added looking for a new entry in the top section. It can now contain:

<MaxValues>
<init name="NumberOfEffects" value="3" />
<init name="MaximumKeyframes" value="10" />
<init name="MaxEntriesInList" value="4" />
<init name="MaxEffectsPerListEntry" value="10" />
<init name="MaxParametersToChange" value="5" />
<init name="KeyframeOffsetMS" value="1" />
</MaxValues>

So there's now a new entry you can use. If it's missing, it will use frames as originally defined. If you add this line:

<init name="KeyframeOffsetMS" value="1" />

Then it will use Milliseconds instead of frames. So if you wanted something at -1 second from the end, you would use -1000 as the value now instead of -30 or -24 or -25 and if you changed the project framerate you will then always get 1 second instead of 1 second +/- 6 frames.