I hope this is what you are looking for:
data=[23 20 18 50 17 24 56 35 38 19 29 53 26]';
f=sort(data,'descend');
f2=reshape([f flipud(f)]',[],1);
vector80=f2(1:round(numel(data)*0.8))
vector20=f2(round(numel(data)*0.8)+1:numel(data))
Another interpretation of what I understood could be like this too:
data=[23 20 18 50 17 24 56 35 38 19 29 53 26]';
f=sort(data,'descend');
vector80=f(1:round(numel(data)*0.8))
vector20=f(round(numel(data)*0.8)+1:numel(data))
