Your vlookup is not returning a value when j = 634. You will need a condition to deal with this when it happens.
For example:
load variables
k = 1; % loop variable
content = zeros(size(r)); % preallocate
for j = 1:length(r)
v = vlookup(AC, r(j), 2, 1);
if ~isempty(v)
content(k) = v;
k = k+1; % increment k
end
end
% remove excess rows that arose due to empty output from vlookup
idel = find(content~=0,1,'last');
content(idel+1:end) = [];
