How can I combine alpha and color in one legend

Question

I have a stacked bar graph where colors represent the category and I've adjusted the alpha to subdivide those into 2. The legend shows the alpha (in shades of grey) and the colors. However I would like to make a legend that contains the combinations.

To combine them I've looked at this question but I cannot combine the alpha and fill. Here is an reproducible figure that doesn't work:

mtcars %>% 
ggplot(aes(gear, mpg, fill = as.factor(vs), alpha = as.factor(am)))+
geom_bar(stat = "identity")+
scale_fill_manual(name = "legend",
                  values = c(
                      "0" = "red",
                      "1" = "blue",
                      "0"="red",
                      "1"="blue"
                      ),
                  labels = c("V-engine, automatic", 
                             "V-engine, manual",
                             "Straight-engine, automatic",
                             "Straight-engine, manual")
                  )+
scale_alpha_manual(name = "legend",
                   values = c(
                       "0" = 1,
                       "1"=2/5,
                       "0"=1,
                       "1"=2/5
                       ),
                   labels = c(
                       "V-engine, automatic", 
                       "V-engine, manual",
                       "Straight-engine, automatic",
                       "Straight-engine, manual"
                       )  )

Answer 1

I approached your question not using aes(alpha) but treating alpha as fill.colour with combination of vs and am.

mtcars %>% 
  ggplot(aes(gear, mpg, fill = interaction(as.factor(vs), as.factor(am)))) +
    geom_bar(stat = "identity") +
    scale_fill_manual(name = "legend",
                      values = c(
                          "0.0" = alpha("red", 1),
                          "1.0" = alpha("blue", 1),
                          "0.1" = alpha("red", 2/5),
                          "1.1" = alpha("blue", 2/5)
                          ),
                      labels = c("V-engine, automatic", 
                                 "V-engine, manual",
                                 "Straight-engine, automatic",
                                 "Straight-engine, manual")
                      )