A leak at almost any location in a VE-pump diesel's fuel system can create a problem, since it can open up the possibility of fuel siphoning back to the tank through the supply line and air being admitted in through the leak in the return hose when the car is not running, causing a loss of prime. However, in your case, there's little chance that this is an issue... Unlike leaks in the inlet line, IP return line leaks will make themselves known with considerable external fuel leakage when the engine is running. On a Volvo particularly, even a small return line leak can make a very big mess in a very short time... ask how I found that out.
If you do not have evidence of fuel leakage under the hood, your return lines are OK, even if they look old and tired.
Vacuum hoses also would not be a factor here, though if yours are in really bad shape you may want to replace them soon after you get the car back on the road, to avoid the possibility of a sudden loss of brake assist that could lead to an unsafe situation.
IIRC both the supply and return hoses are 1/4" ID, at least for a 700 series.... though now that I think about it, I think it may be the case that the 200 series cars did use 5/16" on the fuel inlet, as well as 1983 and some 1984 D24T 700's. Is your fuel filter head inlet attachment a straight fitting that points directly out towards the driver's inner fender, or is it a banjo fitting that points towads the rear of the car? If it is the straight fitting, it is 5/16"; if it has the banjo it is 1/4".
George