R tm package vcorpus: Error in converting corpus to data frame

Your corpus is really just a character vector with some extra attributes. So it's best to convert it to character, then you can save that to a data.frame like so:

library(tm)
x <- c("Hello. Sir!","Tacos? On Tuesday?!?")
mycorpus <- Corpus(VectorSource(x))
mycorpus <- tm_map(mycorpus, removePunctuation)

dataframe <- data.frame(text=unlist(sapply(mycorpus, `[`, "content")), 
    stringsAsFactors=F)

which returns

              text
1        Hello Sir
2 Tacos On Tuesday

UPDATE: With newer version of tm, they seem to have updated the as.list.SimpleCorpus method which really messes with using sapplyand lapply. Now I guess you'd have to use

dataframe <- data.frame(text=sapply(mycorpus, identity), 
    stringsAsFactors=F)

The Corpus classed objected has a content attribute accessible through get:

library("tm")

x <- c("Hello. Sir!","Tacos? On Tuesday?!?")
mycorpus <- Corpus(VectorSource(x))
mycorpus <- tm_map(mycorpus, removePunctuation)

attributes(mycorpus)
# $names
# [1] "content" "meta"    "dmeta"  
# 
# $class
# [1] "SimpleCorpus" "Corpus"      
# 

df <- data.frame(text = get("content", mycorpus))

head(df)
#               text
# 1        Hello Sir
# 2 Tacos On Tuesday

You can convert to data.frame, sort the most frequent words and plot in a wordcloud!

library(tm)
library("wordcloud")
library("RColorBrewer")

x <- c("Hello. Sir!","Tacos? On Tuesday?!?", "Hello")
mycorpus <- Corpus(VectorSource(x))
mycorpus <- tm_map(mycorpus, removePunctuation)

dtm <- TermDocumentMatrix(mycorpus)
m <- as.matrix(dtm)
v <- sort(rowSums(m),decreasing=TRUE)
d <- data.frame(word = names(v),freq=v)
head(d, 10)

#           word freq
#hello     hello    2
#sir         sir    1
#tacos     tacos    1
#tuesday tuesday    1

#plot in a wordcloud
set.seed(1234)
wordcloud(words = d$word, freq = d$freq, min.freq = 1,
          max.words=200, random.order=FALSE, rot.per=0.35, 
          colors=brewer.pal(8, "Dark2"))

The older answer posted by MrFlick works only in previous version on tm, I was able to fix it by removing content from the formula.

dataframe<-data.frame(text=unlist(sapply(mycorpus, `[`)), stringsAsFactors=F)