You're right that it's tracking only the final page, but it doesn't mean your funnel is broken. You've run into an aspect of Google Analytics that is confusingly named. "Required" in a destination goal funnel does not mean "required for the goal to convert", it only means "required for the conversion to show in the Conversions > Goals > Funnel Visualization report".
Google Optimize doesn't need a funnel in order to assign conversions of the goal to each version of the page, however. That may be everything you need.
You can see more details through the funnel for each beginning page, but the trade-off is that your total goal conversion rate will be inaccurate. To get the details, create separate destination goals with the same destination but different funnels: Goal A beginning with
/vsla and Goal B with
/vslb. Mark the first step "required" in both cases.
Goals A and B will count the same conversions, but in the Funnel Visualization report you can separate them by funnel. With the Goal Option drop-down set to Goal A, you will see only conversions which were preceded by a visit to
/vsla. It will give you great information about abandonment rates at each step along the funnel. Likewise you can use Goal B to learn about the performance of
With more detail: The Funnel Visualization report includes all sessions that begin the funnel, whether or not they complete it (if the first step of the funnel is not marked "required", it includes all sessions that hit any page of the funnel). It has 3 columns. The middle column shows the funnel itself: you can see the percentage of page visits that continue on to the next page of the funnel. The final column will show you where you are losing people to when they abandon the funnel. The first column shows you where people were immediately before entering the funnel.
It doesn't seem likely to me that this will tell you much more than the overall conversion rate for each page does, since abandonment after getting past the first page isn't likely to vary between the two funnels by much. But it is an option!