You can do something along the lines of
<block myblock>
/ trials = [1-8 = noreplace(vowel, consonant)]
</block>
<trial vowel>
/ stimulustimes = [0=vowel; 1500=blank; 2000=blank]
/ validresponse = ("v", "c")
/ correctresponse = ("v")
/ beginresponsetime = 0
/ responseinterrupt = frames
/ trialduration = 2000
</trial>
<trial consonant>
/ stimulustimes = [0=consonant; 1500=blank; 2000=blank]
/ validresponse = ("v", "c")
/ correctresponse = ("c")
/ beginresponsetime = 0
/ responseinterrupt = frames
/ trialduration = 2000
</trial>
<text vowel>
/ items = vowels
/ erase = false
</text>
<item vowels>
/ 1 = "A"
/ 2 = "E"
/ 3 = "I"
/ 4 = "O"
</item>
<text consonant>
/ items = consonants
/ erase = false
</text>
<item consonants>
/ 1 = "B"
/ 2 = "F"
/ 3 = "J"
/ 4 = "P"
</item>
<shape blank>
/ color = white
/ shape = rectangle
/ size = (100%, 100%)
/ erase = false
</shape>
(works the same with <picture> instead of <text> stimuli).