Remember in the File menu down to Page Options, with that opened click on the View tab, then in the middle at Bitmaps, click on the box which is labeled "DPI when auto generated" and type in 300. Most import that you do that as you might miss when you select your vectors and do a "Ctrl+Shift+C" to create your bitmap copy as it will 96. When the window opens and you see the dialogue make sure that that you see PNG at the top. If it is on the palette options you must see "True colour +Alpha". Alongside the palette you will see Bitmap Size, click on that just to make sure that in DPI that 300 is selected. The default is 96 dpi which is useless for print so go at least 150 but 300 is better.